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/playback_reporting.rs

@@ -24,6 +24,7 @@ pub struct PlaybackReporterWrapper(pub Arc<TokioMutex<Option<PlaybackReporter>>>
 
 /// Initialize playback reporter (called after login)
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_reporter_init(
     reporter_wrapper: State<'_, PlaybackReporterWrapper>,
     db: State<'_, DatabaseWrapper>,
@@ -66,6 +67,7 @@ pub async fn playback_reporter_init(
 
 /// Destroy playback reporter (called on logout)
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_reporter_destroy(
     reporter_wrapper: State<'_, PlaybackReporterWrapper>,
 ) -> Result<(), String> {
@@ -76,6 +78,7 @@ pub async fn playback_reporter_destroy(
 
 /// Report playback start
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_report_start(
     reporter: State<'_, PlaybackReporterWrapper>,
     connectivity: State<'_, ConnectivityMonitorWrapper>,
@@ -110,6 +113,7 @@ pub async fn playback_report_start(
 
 /// Report playback progress
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_report_progress(
     reporter: State<'_, PlaybackReporterWrapper>,
     connectivity: State<'_, ConnectivityMonitorWrapper>,
@@ -138,6 +142,7 @@ pub async fn playback_report_progress(
 
 /// Report playback stopped
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_report_stopped(
     reporter: State<'_, PlaybackReporterWrapper>,
     connectivity: State<'_, ConnectivityMonitorWrapper>,
@@ -164,6 +169,7 @@ pub async fn playback_report_stopped(
 
 /// Mark item as played
 #[tauri::command]
+#[specta::specta]
 pub async fn playback_mark_played(
     reporter: State<'_, PlaybackReporterWrapper>,
     connectivity: State<'_, ConnectivityMonitorWrapper>,