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/sessions.rs

@@ -10,6 +10,7 @@ pub struct SessionPollerWrapper(pub Arc<SessionPollerManager>);
 
 /// Set polling frequency hint based on UI state
 #[tauri::command]
+#[specta::specta]
 pub fn sessions_set_polling_hint(
     poller: State<'_, SessionPollerWrapper>,
     hint: String,
@@ -27,6 +28,7 @@ pub fn sessions_set_polling_hint(
 
 /// Manually trigger a session poll (for refresh button)
 #[tauri::command]
+#[specta::specta]
 pub async fn sessions_poll_now(
     poller: State<'_, SessionPollerWrapper>,
 ) -> Result<Vec<SessionInfo>, String> {