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/download/pinning.rs

@@ -8,6 +8,7 @@ use crate::storage::db_service::{DatabaseService, Query, QueryParam};
 
 /// Pin an item's metadata (protects from cache clear)
 #[tauri::command]
+#[specta::specta]
 pub async fn pin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> Result<(), String> {
     let db_service = {
         let database = db.0.lock().map_err(|e| e.to_string())?;
@@ -25,6 +26,7 @@ pub async fn pin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> Result
 
 /// Unpin an item's metadata
 #[tauri::command]
+#[specta::specta]
 pub async fn unpin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> Result<(), String> {
     let db_service = {
         let database = db.0.lock().map_err(|e| e.to_string())?;
@@ -42,6 +44,7 @@ pub async fn unpin_item(db: State<'_, DatabaseWrapper>, item_id: String) -> Resu
 
 /// Check if an item is pinned
 #[tauri::command]
+#[specta::specta]
 pub async fn is_item_pinned(db: State<'_, DatabaseWrapper>, item_id: String) -> Result<bool, String> {
     let db_service = {
         let database = db.0.lock().map_err(|e| e.to_string())?;