Workstream E (backend): wire tauri-specta — annotate commands, derive Type, generate-ready Builder

- Add #[specta::specta] to all 201 #[tauri::command] functions.
- Derive specta::Type on all IPC DTOs (repository/types, settings, player/storage/
  download command DTOs, player enums, jellyfin SessionInfo/NowPlayingItem/PlayState,
  ThumbnailCacheStats, DownloadInfo, CacheConfig, etc.).
- Replace tauri::generate_handler! with a tauri_specta::Builder + collect_commands!
  in lib.rs (exports bindings.ts in debug builds).

Two contract changes required by specta constraints (frontend migration follows):
- specta caps command arity at 10 args: download_item_and_start / download_item /
  download_video now take a single request struct (params bundled, body unchanged
  via destructuring).
- specta can't parse split serde rename_all: SessionInfo/NowPlayingItem/PlayState
  switched to rename_all = "PascalCase" (Jellyfin deserialization preserved; these
  now serialize PascalCase to the frontend).

cargo check --lib is clean (0 errors). Frontend migration to bindings.ts is the next step.

src-tauri/src/player/media.rs

@@ -3,7 +3,7 @@ use std::path::PathBuf;
 
 /// Context for the current queue - where did the queue items come from?
 /// This is used for remote playback transfer to send album/playlist context.
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Default)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize, PartialEq, Default)]
 #[serde(tag = "type", rename_all = "lowercase")]
 pub enum QueueContext {
     /// Playing from a specific album
@@ -23,7 +23,7 @@ pub enum QueueContext {
 }
 
 /// Represents a subtitle track
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct SubtitleTrack {
     /// Stream index in the media source
     pub index: i32,
@@ -40,7 +40,7 @@ pub struct SubtitleTrack {
 /// Represents a media item that can be played
 ///
 /// TRACES: UR-003, UR-004 | DR-002
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "camelCase")]
 pub struct MediaItem {
     /// Unique identifier
@@ -106,7 +106,7 @@ pub struct MediaItem {
     pub server_id: Option<String>,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "lowercase")]
 pub enum MediaType {
     Audio,
@@ -114,7 +114,7 @@ pub enum MediaType {
 }
 
 /// TRACES: UR-002, UR-003, UR-004, UR-011 | DR-003
-#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize, PartialEq)]
 #[serde(tag = "type", rename_all = "lowercase")]
 pub enum MediaSource {
     /// Streaming from Jellyfin server