jellytau/docs/architecture/01-rust-backend.md

# Rust Backend Architecture

**Location**: `src-tauri/src/`

## Media Session State Machine

**Location**: `src-tauri/src/player/session.rs`

The media session tracks the high-level playback context (what kind of media is being consumed) and persists beyond individual playback states. This enables persistent UI (miniplayer for audio) and proper transitions between content types.

**Architecture Note:** The session manager is a separate app-level state manager (not inside PlayerController), coordinated by the commands layer. This maintains clean separation of concerns.

```mermaid
stateDiagram-v2
    [*] --> Idle

    Idle --> AudioActive : play_queue(audio)
    Idle --> MovieActive : play_item(movie)
    Idle --> TvShowActive : play_item(episode)

    state "Audio Session" as AudioSession {
        [*] --> AudioActive
        AudioActive --> AudioInactive : playback_ended
        AudioInactive --> AudioActive : resume/play
        AudioActive --> AudioActive : next/previous
    }

    state "Movie Session" as MovieSession {
        [*] --> MovieActive
        MovieActive --> MovieInactive : playback_ended
        MovieInactive --> MovieActive : resume
    }

    state "TV Show Session" as TvShowSession {
        [*] --> TvShowActive
        TvShowActive --> TvShowInactive : playback_ended
        TvShowInactive --> TvShowActive : next_episode/resume
    }

    AudioSession --> Idle : dismiss/clear_queue
    AudioSession --> MovieSession : play_item(movie)
    AudioSession --> TvShowSession : play_item(episode)

    MovieSession --> Idle : dismiss/playback_complete
    MovieSession --> AudioSession : play_queue(audio)

    TvShowSession --> Idle : dismiss/series_complete
    TvShowSession --> AudioSession : play_queue(audio)

    note right of Idle
        No active media session
        Queue may exist but not playing
        No miniplayer/video player shown
    end note

    note right of AudioSession
        SHOW: Miniplayer (always visible)
        - Active: Play/pause/skip controls enabled
        - Inactive: Play button to resume queue
        Persists until explicit dismiss
    end note

    note right of MovieSession
        SHOW: Full video player
        - Active: Video playing/paused
        - Inactive: Resume dialog
        Auto-dismiss when playback ends
    end note

    note right of TvShowSession
        SHOW: Full video player + Next Episode UI
        - Active: Video playing/paused
        - Inactive: Next episode prompt
        Auto-dismiss when series ends
    end note
```

**Session State Enum:**
```rust
#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
#[serde(tag = "type", rename_all = "snake_case")]
pub enum MediaSessionType {
    /// No active session - browsing library
    Idle,

    /// Audio playback session (music, audiobooks, podcasts)
    /// Persists until explicitly dismissed
    Audio {
        /// Last/current track being played
        last_item: Option<MediaItem>,
        /// True = playing/paused, False = stopped/ended
        is_active: bool,
    },

    /// Movie playback (single video, auto-dismiss on end)
    Movie {
        item: MediaItem,
        is_active: bool,  // true = playing/paused, false = ended
    },

    /// TV show playback (supports next episode auto-advance)
    TvShow {
        item: MediaItem,
        series_id: String,
        is_active: bool,  // true = playing/paused, false = ended
    },
}
```

**State Transitions & Rules:**

| From State | Event | To State | UI Behavior | Notes |
|------------|-------|----------|-------------|-------|
| Idle | `play_queue(audio)` | Audio (active) | Show miniplayer | Creates audio session |
| Idle | `play_item(movie)` | Movie (active) | Show video player | Creates movie session |
| Idle | `play_item(episode)` | TvShow (active) | Show video player | Creates TV session |
| Audio (active) | `playback_ended` | Audio (inactive) | Miniplayer stays visible | Queue preserved |
| Audio (inactive) | `play/resume` | Audio (active) | Miniplayer enabled | Resume from queue |
| Audio (active/inactive) | `dismiss` | Idle | Hide miniplayer | Clear session |
| Audio (active/inactive) | `play_item(movie)` | Movie (active) | Switch to video player | Replace session |
| Movie (active) | `playback_ended` | Idle | Hide video player | Auto-dismiss |
| Movie (active) | `dismiss` | Idle | Hide video player | User dismiss |
| TvShow (active) | `playback_ended` | TvShow (inactive) | Show next episode UI | Wait for user choice |
| TvShow (inactive) | `next_episode` | TvShow (active) | Play next episode | Stay in session |
| TvShow (inactive) | `series_complete` | Idle | Hide video player | No more episodes |

**Key Design Decisions:**

1. **Audio Sessions Persist**: Miniplayer stays visible even when queue ends, allows easy resume
2. **Video Sessions Auto-Dismiss**: Movies auto-close when finished (unless paused)
3. **Single Active Session**: Playing new content type replaces current session
4. **Explicit Dismiss for Audio**: User must click close button to clear audio session
5. **Session != PlayerState**: Session is higher-level, PlayerState tracks playing/paused/seeking

**Edge Cases Handled:**

- Album finishes: Session goes inactive, miniplayer shows last track with play disabled
- User wants to dismiss: Close button clears session -> Idle
- Switch content types: New session replaces old (audio -> movie)
- Paused for extended time: Session persists indefinitely
- Playback errors: Session stays inactive, allows retry
- Queue operations while idle: Queue exists but no session created until play

## Player State Machine (Low-Level Playback)

**Location**: `src-tauri/src/player/state.rs`

The player uses a deterministic state machine with 6 states (operates within a media session):

```mermaid
stateDiagram-v2
    [*] --> Idle
    Idle --> Loading : Load
    Loading --> Playing : MediaLoaded
    Playing --> Paused : Pause
    Paused --> Playing : Play
    Paused --> Seeking : Seek
    Seeking --> Playing : PositionUpdate
    Playing --> Idle : Stop
    Paused --> Idle : Stop
    Idle --> Error : Error
    Loading --> Error : Error
    Playing --> Error : Error
    Paused --> Error : Error
    Seeking --> Error : Error

    state Playing {
        [*] : position, duration
    }
    state Paused {
        [*] : position, duration
    }
    state Seeking {
        [*] : target
    }
    state Error {
        [*] : error message
    }
```

**State Enum:**
```rust
pub enum PlayerState {
    Idle,
    Loading { media: MediaItem },
    Playing { media: MediaItem, position: f64, duration: f64 },
    Paused { media: MediaItem, position: f64, duration: f64 },
    Seeking { media: MediaItem, target: f64 },
    Error { media: Option<MediaItem>, error: String },
}
```

**Event Enum:**
```rust
pub enum PlayerEvent {
    Load(MediaItem),
    Play,
    Pause,
    Stop,
    Seek(f64),
    Next,
    Previous,
    MediaLoaded(f64),      // duration
    PositionUpdate(f64),   // position
    PlaybackEnded,
    Error(String),
}
```

## Playback Mode State Machine

**Location**: `src-tauri/src/playback_mode/mod.rs`

The playback mode manages whether media is playing locally on the device or remotely on another Jellyfin session (TV, browser, etc.):

```mermaid
stateDiagram-v2
    [*] --> Idle

    Idle --> Local : play_queue()
    Idle --> Remote : transfer_to_remote(session_id)

    Local --> Remote : transfer_to_remote(session_id)
    Local --> Idle : stop()

    Remote --> Local : transfer_to_local()
    Remote --> Idle : session_disconnected()
    Remote --> Idle : stop()

    state Local {
        [*] : Playing on device
        [*] : ExoPlayer active
        [*] : Volume buttons -> device
    }

    state Remote {
        [*] : Controlling session
        [*] : session_id
        [*] : Volume buttons -> remote
        [*] : Android: VolumeProvider active
    }

    state Idle {
        [*] : No active playback
    }
```

**State Enum:**
```rust
pub enum PlaybackMode {
    Local,                           // Playing on local device
    Remote { session_id: String },   // Controlling remote Jellyfin session
    Idle,                            // No active playback
}
```

**State Transitions:**

| From | Event | To | Side Effects |
|------|-------|-----|----|
| Idle | `play_queue()` | Local | Start local playback |
| Idle | `transfer_to_remote(session_id)` | Remote | Send queue to remote session |
| Local | `transfer_to_remote(session_id)` | Remote | Stop local, send queue to remote, enable remote volume (Android) |
| Local | `stop()` | Idle | Stop local playback |
| Remote | `transfer_to_local()` | Local | Get remote state, stop remote, start local at same position, disable remote volume |
| Remote | `stop()` | Idle | Stop remote playback, disable remote volume |
| Remote | `session_disconnected()` | Idle | Session lost, disable remote volume |

**Integration with Player State Machine:**

- When `PlaybackMode = Local`: Player state machine is active (Idle/Loading/Playing/Paused/etc.)
- When `PlaybackMode = Remote`: Player state is typically Idle (remote session controls playback)
- When `PlaybackMode = Idle`: Player state is Idle

**Android Volume Control Integration:**

When transitioning to `Remote` mode on Android:
1. Call `enable_remote_volume(initial_volume)`
2. VolumeProviderCompat intercepts hardware volume buttons
3. PlaybackStateCompat is set to STATE_PLAYING (shows volume UI)
4. Volume commands routed to remote session via Jellyfin API

When transitioning away from `Remote` mode:
1. Call `disable_remote_volume()`
2. Volume buttons return to controlling device volume
3. PlaybackStateCompat set to STATE_NONE
4. VolumeProviderCompat is cleared

## Media Item & Source

**Location**: `src-tauri/src/player/media.rs`

```rust
pub struct MediaItem {
    pub id: String,
    pub title: String,
    pub artist: Option<String>,
    pub album: Option<String>,
    pub duration: Option<f64>,
    pub artwork_url: Option<String>,
    pub media_type: MediaType,
    pub source: MediaSource,
}

pub enum MediaType {
    Audio,
    Video,
}

pub enum MediaSource {
    Remote {
        stream_url: String,
        jellyfin_item_id: String,
    },
    Local {
        file_path: PathBuf,
        jellyfin_item_id: Option<String>,
    },
    DirectUrl {
        url: String,
    },
}
```

The `MediaSource` enum enables:
- **Remote**: Streaming from Jellyfin server
- **Local**: Downloaded/cached files (future offline support)
- **DirectUrl**: Direct URLs (channel plugins, external sources)

## Queue Manager

**Location**: `src-tauri/src/player/queue.rs`

```rust
pub struct QueueManager {
    items: Vec<MediaItem>,
    current_index: Option<usize>,
    shuffle: bool,
    repeat: RepeatMode,
    shuffle_order: Vec<usize>,  // Fisher-Yates permutation
    history: Vec<usize>,        // For back navigation in shuffle
}

pub enum RepeatMode {
    Off,
    All,
    One,
}
```

**Queue Navigation Logic:**

```mermaid
flowchart TB
    QM[QueueManager]
    QM --> Shuffle
    QM --> Repeat
    QM --> History

    subgraph Shuffle["Shuffle Mode"]
        ShuffleOff["OFF<br/>next() returns index + 1"]
        ShuffleOn["ON<br/>next() follows shuffle_order[]"]
    end

    subgraph Repeat["Repeat Mode"]
        RepeatOff["OFF<br/>next() at end: -> None"]
        RepeatAll["ALL<br/>next() at end: -> wrap to index 0"]
        RepeatOne["ONE<br/>next() returns same item"]
    end

    subgraph History["History"]
        HistoryDesc["Used for previous()<br/>in shuffle mode"]
    end
```

## Favorites System

**Location**:
- Commands: `src-tauri/src/commands/favorites.rs` (offline drain),
  `src-tauri/src/commands/repository.rs` (query + toggle),
  `src-tauri/src/commands/storage/` (local `user_data` writes)
- Repository: `get_favorites` on the trait, implemented by `online.rs`,
  `offline.rs` and `hybrid.rs`
- Frontend: `src/lib/services/favorites.ts`,
  `src/lib/components/FavoriteButton.svelte`, `/library/favorites`

Favouriting has two halves that are easy to confuse: **marking** an item, which
has existed since UR-017, and **browsing** what was marked, which arrived with
UR-067…069 (DR-113 … DR-120). Both go through the repository, not around it.

### Marking

Optimistic local write, then server sync:

```mermaid
flowchart TB
    UI[FavoriteButton] -->|Click| Service[toggleFavorite]
    Service -->|"1. Optimistic"| LocalDB[("SQLite user_data<br/>is_favorite, pending_sync")]
    Service -->|"2. Sync"| Repo[Repository]
    Repo -->|POST / DELETE| JellyfinAPI["/Users/{id}/FavoriteItems/{itemId}"]
    Service -->|"3. Mark synced"| LocalDB
    Drain["spawn_favorites_drain<br/>(background task)"] -->|"pending_sync = 1"| Repo
```

1. The local row is updated immediately, so the heart fills without a round trip.
2. The repository is asked to mark or unmark on the server.
3. On success `pending_sync` is cleared; on failure the row stays pending.
4. A **background drain** (`spawn_favorites_drain`, started in `lib.rs` setup)
   retries pending rows, so a favourite marked offline still reaches the server
   (DR-120). This is the same pattern as the sync-queue drain — see
   [Background workers](#background-workers).

### Browsing

`get_favorites(scope, options)` answers "what did this user favourite", across
libraries, with the **scope owned by Rust** — the frontend sends a
[`SearchScope`](#search-scope-and-the-taxonomy-boundary) variant and never names
an item type. `HybridRepository` splits it the same way it splits every query:

| Method | Used for |
|--------|----------|
| `get_favorites_cache_only` | The instant leg — the local `user_data` join |
| `get_favorites_server_only` | The reconciliation leg |
| `get_favorites` | Cache-first with server merge, per the repository's usual policy |

`GetItemsOptions.favorites_only` is the other entry point: it filters an
*existing* library listing rather than starting a cross-library query (DR-116),
which is what a library page's favourites filter uses.

Server favourite state is mirrored into the local `user_data` table on catalog
sync (DR-113/DR-114), so a favourite marked in another Jellyfin client shows up
here — before this, `MediaItem.user_data` was left empty and no query anywhere
asked for favourites.

**Tauri commands**:

| Command | Description |
|---------|-------------|
| `repository_get_favorites` | Cross-library favourites for a scope |
| `repository_mark_favorite` / `repository_unmark_favorite` | Toggle on the server, through the repository |
| `storage_toggle_favorite` | Local optimistic write (`is_favorite`, `pending_sync`) |
| `storage_mark_synced` | Clear `pending_sync` after a successful server write |

**Frontend surfaces** (DR-117 … DR-119): the `/library/favorites` page with a
scope selector, favourite rows on home (`favoriteMovies` / `favoriteShows` /
`favoriteMusic` in `stores/home.ts`), a favourites tile per category in the
library mosaic, and `FavoriteButton` mounted wherever a whole item is shown —
movie, series, episode, album, artist and playlist detail views as well as the
mini player.

## Player Backend Trait

**Location**: `src-tauri/src/player/backend.rs`

```rust
pub trait PlayerBackend: Send + Sync {
    fn load(&mut self, media: &MediaItem) -> Result<(), PlayerError>;
    fn play(&mut self) -> Result<(), PlayerError>;
    fn pause(&mut self) -> Result<(), PlayerError>;
    fn stop(&mut self) -> Result<(), PlayerError>;
    fn seek(&mut self, position: f64) -> Result<(), PlayerError>;
    fn set_volume(&mut self, volume: f32) -> Result<(), PlayerError>;
    fn position(&self) -> f64;
    fn duration(&self) -> Option<f64>;
    fn state(&self) -> PlayerState;
    fn is_loaded(&self) -> bool;
    fn volume(&self) -> f32;
}
```

**Implementations:**
- `NullBackend` - Mock backend for testing
- `MpvBackend` - Linux playback via libmpv (see [05-platform-backends.md](05-platform-backends.md))
- `ExoPlayerBackend` - Android playback via ExoPlayer/Media3 (see [05-platform-backends.md](05-platform-backends.md))

## Player Controller

**Location**: `src-tauri/src/player/mod.rs`

The `PlayerController` orchestrates playback:

```rust
pub struct PlayerController {
    backend: Arc<Mutex<Box<dyn PlayerBackend>>>,
    queue: Arc<Mutex<QueueManager>>,
    muted: bool,
    sleep_timer: Arc<Mutex<SleepTimerState>>,
    autoplay_settings: Arc<Mutex<AutoplaySettings>>,
    autoplay_episode_count: Arc<Mutex<u32>>,  // Session-based counter
    repository: Arc<Mutex<Option<Arc<dyn MediaRepository>>>>,
    event_emitter: Arc<Mutex<Option<Arc<dyn PlayerEventEmitter>>>>,
    // ... other fields
}
```

**Key Methods:**
- `play_item(item)`: Load and play single item (resets autoplay counter)
- `play_queue(items, start_index)`: Load queue and start playback (resets autoplay counter)
- `next()` / `previous()`: Queue navigation (resets autoplay counter)
- `toggle_shuffle()` / `cycle_repeat()`: Mode changes
- `set_sleep_timer(mode)` / `cancel_sleep_timer()`: Sleep timer control
- `on_playback_ended()`: Autoplay decision making (checks sleep timer, episode limit, queue)

## Playlist System

**Location**: `src-tauri/src/commands/playlist.rs`, `src-tauri/src/repository/`

**TRACES**: UR-014 | JA-019 | JA-020

The playlist system provides full CRUD operations for Jellyfin playlists with offline support through the cache-first repository pattern.

**Types:**

```rust
/// A media item within a playlist, with its distinct playlist entry ID
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct PlaylistEntry {
    /// Jellyfin's PlaylistItemId (distinct from the media item ID)
    pub playlist_item_id: String,
    #[serde(flatten)]
    pub item: MediaItem,
}

/// Result of creating a new playlist
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(rename_all = "camelCase")]
pub struct PlaylistCreatedResult {
    pub id: String,
}
```

**Key Design Decision**: `PlaylistEntry` wraps a `MediaItem` with a distinct `playlist_item_id`. This is critical because removing items from a playlist requires the playlist entry ID (not the media item ID), since the same track can appear multiple times.

**MediaRepository Trait Methods:**
```rust
async fn create_playlist(&self, name: &str, item_ids: Option<Vec<String>>) -> Result<PlaylistCreatedResult, RepoError>;
async fn delete_playlist(&self, playlist_id: &str) -> Result<(), RepoError>;
async fn rename_playlist(&self, playlist_id: &str, name: &str) -> Result<(), RepoError>;
async fn get_playlist_items(&self, playlist_id: &str) -> Result<Vec<PlaylistEntry>, RepoError>;
async fn add_to_playlist(&self, playlist_id: &str, item_ids: Vec<String>) -> Result<(), RepoError>;
async fn remove_from_playlist(&self, playlist_id: &str, entry_ids: Vec<String>) -> Result<(), RepoError>;
async fn move_playlist_item(&self, playlist_id: &str, item_id: &str, new_index: u32) -> Result<(), RepoError>;
```

**Cache Strategy:**
- **Write operations** (create, delete, rename, add, remove, move): Delegate directly to online repository
- **Read operation** (`get_playlist_items`): Uses cache-first parallel racing (100ms cache timeout, server fallback)
- Background cache update after server fetch via `save_playlist_items_to_cache()`

**Playlist Tauri Commands:**

| Command | Parameters | Returns |
|---------|------------|---------|
| `playlist_create` | `handle, name, item_ids?` | `PlaylistCreatedResult` |
| `playlist_delete` | `handle, playlist_id` | `()` |
| `playlist_rename` | `handle, playlist_id, name` | `()` |
| `playlist_get_items` | `handle, playlist_id` | `Vec<PlaylistEntry>` |
| `playlist_add_items` | `handle, playlist_id, item_ids` | `()` |
| `playlist_remove_items` | `handle, playlist_id, entry_ids` | `()` |
| `playlist_move_item` | `handle, playlist_id, item_id, new_index` | `()` |

## Tauri Commands (Player)

**Location**: `src-tauri/src/commands/player.rs`

| Command | Parameters | Returns |
|---------|------------|---------|
| `player_play_item` | `PlayItemRequest` | `PlayerStatus` |
| `player_play_queue` | `items, start_index, shuffle` | `PlayerStatus` |
| `player_play` | - | `PlayerStatus` |
| `player_pause` | - | `PlayerStatus` |
| `player_toggle` | - | `PlayerStatus` |
| `player_stop` | - | `PlayerStatus` |
| `player_next` | - | `PlayerStatus` |
| `player_previous` | - | `PlayerStatus` |
| `player_seek` | `position: f64` | `PlayerStatus` |
| `player_set_volume` | `volume: f32` | `PlayerStatus` |
| `player_toggle_shuffle` | - | `QueueStatus` |
| `player_cycle_repeat` | - | `QueueStatus` |
| `player_get_status` | - | `PlayerStatus` |
| `player_get_queue` | - | `QueueStatus` |
| `player_get_session` | - | `MediaSessionType` |
| `player_dismiss_session` | - | `()` |
| `player_set_sleep_timer` | `mode: SleepTimerMode` | `()` |
| `player_cancel_sleep_timer` | - | `()` |
| `player_set_video_settings` | `settings: VideoSettings` | `VideoSettings` |
| `player_get_video_settings` | - | `VideoSettings` |
| `player_set_autoplay_settings` | `settings: AutoplaySettings` | `AutoplaySettings` |
| `player_get_autoplay_settings` | - | `AutoplaySettings` |
| `player_on_playback_ended` | - | `()` |

## Domain Vocabulary Owned by Rust

The frontend is presentation-only and must not encode Jellyfin's *taxonomy* — the
rule in [CLAUDE.md](../../CLAUDE.md) and
[scoped-search-boundary.md](../specs/scoped-search-boundary.md). These are the
places where that vocabulary actually lives.

### Search scope and the taxonomy boundary

**Location**: `src-tauri/src/repository/types.rs`

`SearchScope` is the canonical example the boundary rule is taught from. The
frontend sends an opaque variant; Rust expands it into Jellyfin item types:

```rust
pub enum SearchScope { All, Music, Movies, Tv }

impl SearchScope {
    /// The Jellyfin item types this scope requests, or `None` for `All`.
    pub fn item_types(self) -> Option<Vec<String>> { … }

    /// The scope a library of this Jellyfin `CollectionType` belongs to.
    pub fn for_collection_type(collection_type: &str) -> Option<SearchScope> { … }
}
```

Two details that are load-bearing:

- `All` returns `None`, **not** the union of every listed type. An explicit
  `includeItemTypes` list filters out anything not named in it, so a union would
  silently drop People, folders, and any type nobody enumerated. Callers must
  omit the filter entirely on `None`.
- `for_collection_type` maps a Jellyfin `CollectionType` to a favourites
  category (DR-175). It changes when *Jellyfin* renames a collection type, not
  when the library page is redesigned — which is the test for whether something
  belongs on this side of the boundary.

⚠️ **The result side has not moved yet.** `GROUP_ITEM_TYPES` in
`src/lib/utils/searchScope.ts` still maps result groups to item types in the
frontend, and `check:boundary` does not match its shape. Tracked as Stage 2 of
[scoped-search-boundary-implementation.md](../specs/scoped-search-boundary-implementation.md).

### Library exclusions

**Location**: `src-tauri/src/repository/exclusions.rs` (TRACES: UR-076 | DR-209)

Folders the user has chosen to keep out of music browsing — a "Podcasts" folder
inside a music library being the canonical case. Excluded **by item id**, not by
name, in a process-wide `RwLock<Vec<String>>` restored from the database at
startup, and applied by the repository layer to every music query (libraries,
artists, albums, genres, search, home rows).

The id is normalised (`trim`, strip `-`, lowercase) because Jellyfin writes the
same GUID both dashed and undashed depending on the endpoint. The predecessor was
a frontend filter matching the English string "Podcasts" — wrong in three ways at
once, and the reason this lives in the repository.

The set is process-wide rather than a field on a repository for the same reason
as `online::STREAMING_QUALITY`: it is a preference about *this user's browsing*,
not about a server session, so it must survive a repository being rebuilt on
re-login.

### Streaming quality ladder

**Location**: `src-tauri/src/settings.rs` (TRACES: UR-074 | DR-162)

`StreamingQuality` is a bandwidth ladder (`Original`, 20/10/8/4/2/1 Mbps,
720 kbps), not a resolution picker: it exists to fit a connection, and the
resolution cap is chosen *from* the bitrate so the encoder does not spend a small
budget on pixels it cannot afford.

| Method | Answers |
|--------|---------|
| `max_bitrate()` | Total bits/s (video + audio), `None` for `Original` |
| `audio_bitrate()` | The audio share — shrinks down the ladder, so 384 kbps is not a third of the budget at the bottom |
| `video_bitrate()` | Total minus audio, so the two together honour the ceiling |
| `max_height()` | Resolution ceiling that suits the bitrate |

The ceiling goes to `PlaybackInfo` as `MaxStreamingBitrate` **and** into the
device profile. Sending it there — not just on the transcode URL — is what makes
the cap real: a stream the server decides to *direct play* is served at the
source file's own bitrate, and no URL parameter afterwards can reduce it.

The frontend names a variant and nothing else; the labels the picker shows are
served over IPC by `player_get_streaming_qualities`.

## Background workers

Three long-lived tasks are spawned from the Tauri `setup` hook in `lib.rs`. All
three exist because *when* something happens is a backend policy, not something
a page load should decide.

| Worker | Location | Responsibility |
|--------|----------|----------------|
| `spawn_catalog_indexer` | `commands/catalog.rs` | Keeps the local FTS5 catalog fresh (DR-109, IR-030) |
| `spawn_favorites_drain` | `commands/favorites.rs` | Retries favourite toggles made while offline (DR-120) |
| `spawn_sync_queue_drain` | `commands/sync_drain.rs` | Drains the offline mutation queue (DR-131) |

### Catalog indexer

Replaces the frontend's startup-only `syncCatalog()` call. It ticks on
`CATALOG_INDEX_TICK` and runs a pass when three things hold: a repository exists,
the server is reachable, and the index is due per `index_is_due`. A tick is
nearly free — one indexed `app_settings` lookup — which is what makes it
responsive to events it cannot subscribe to, such as signing in: a fresh install
would otherwise sit unindexed until the next scheduled pass.

`index_is_due` treats both "never indexed" and an unparseable stored timestamp as
due; a corrupt timestamp should trigger a re-index, not silently freeze the
catalog. A failed pass is never fatal — it leaves the existing index in place and
warns. Progress is emitted on `CATALOG_INDEX_EVENT` for the staleness hint in the
UI.