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/commands/player/queue.rs

@@ -16,7 +16,7 @@ use crate::repository::types::{ImageOptions, ImageType};
 use crate::repository::MediaRepository;
 
 /// Request to add items to queue
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct AddToQueueRequest {
     pub items: Vec<PlayItemRequest>,
@@ -24,7 +24,7 @@ pub struct AddToQueueRequest {
 }
 
 /// Request to add a track by ID - backend fetches metadata
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct AddTrackByIdRequest {
     pub track_id: String,
@@ -32,7 +32,7 @@ pub struct AddTrackByIdRequest {
 }
 
 /// Request to add multiple tracks by IDs - backend fetches metadata
-#[derive(Debug, Deserialize)]
+#[derive(specta::Type, Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct AddTracksByIdsRequest {
     pub track_ids: Vec<String>,
@@ -40,6 +40,7 @@ pub struct AddTracksByIdsRequest {
 }
 
 #[tauri::command]
+#[specta::specta]
 pub async fn player_add_to_queue(
     player: State<'_, PlayerStateWrapper>,
     db: State<'_, DatabaseWrapper>,
@@ -81,6 +82,7 @@ pub async fn player_add_to_queue(
 }
 
 #[tauri::command]
+#[specta::specta]
 pub async fn player_remove_from_queue(
     player: State<'_, PlayerStateWrapper>,
     index: usize,
@@ -107,6 +109,7 @@ pub async fn player_remove_from_queue(
 }
 
 #[tauri::command]
+#[specta::specta]
 pub async fn player_move_in_queue(
     player: State<'_, PlayerStateWrapper>,
     from_index: usize,
@@ -137,6 +140,7 @@ pub async fn player_move_in_queue(
 
 /// Add a track to queue by ID - backend fetches metadata and constructs URLs
 #[tauri::command]
+#[specta::specta]
 pub async fn player_add_track_by_id(
     player: State<'_, PlayerStateWrapper>,
     db: State<'_, DatabaseWrapper>,
@@ -236,6 +240,7 @@ pub async fn player_add_track_by_id(
 
 /// Add multiple tracks to queue by IDs - backend fetches metadata and constructs URLs
 #[tauri::command]
+#[specta::specta]
 pub async fn player_add_tracks_by_ids(
     player: State<'_, PlayerStateWrapper>,
     db: State<'_, DatabaseWrapper>,
@@ -339,6 +344,7 @@ pub async fn player_add_tracks_by_ids(
 }
 
 #[tauri::command]
+#[specta::specta]
 pub async fn player_skip_to(
     player: State<'_, PlayerStateWrapper>,
     index: usize,