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/sync.rs

@@ -12,7 +12,7 @@ use super::storage::DatabaseWrapper;
 use crate::storage::db_service::{DatabaseService, Query, QueryParam};
 
 /// Sync queue item returned to frontend
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct SyncQueueItem {
     pub id: i64,
@@ -28,6 +28,7 @@ pub struct SyncQueueItem {
 
 /// Queue a mutation for sync to server
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_queue_mutation(
     db: State<'_, DatabaseWrapper>,
     user_id: String,
@@ -59,6 +60,7 @@ pub async fn sync_queue_mutation(
 
 /// Get all pending sync operations for a user
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_get_pending(
     db: State<'_, DatabaseWrapper>,
     user_id: String,
@@ -107,6 +109,7 @@ pub async fn sync_get_pending(
 
 /// Mark a sync operation as in progress
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_mark_processing(
     db: State<'_, DatabaseWrapper>,
     id: i64,
@@ -127,6 +130,7 @@ pub async fn sync_mark_processing(
 
 /// Mark a sync operation as completed
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_mark_completed(
     db: State<'_, DatabaseWrapper>,
     id: i64,
@@ -147,6 +151,7 @@ pub async fn sync_mark_completed(
 
 /// Mark a sync operation as failed with error message
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_mark_failed(
     db: State<'_, DatabaseWrapper>,
     id: i64,
@@ -173,6 +178,7 @@ pub async fn sync_mark_failed(
 
 /// Get count of pending sync operations for a user
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_get_pending_count(
     db: State<'_, DatabaseWrapper>,
     user_id: String,
@@ -195,6 +201,7 @@ pub async fn sync_get_pending_count(
 
 /// Delete completed sync operations older than specified days
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_cleanup_completed(
     db: State<'_, DatabaseWrapper>,
     days_old: i32,
@@ -217,6 +224,7 @@ pub async fn sync_cleanup_completed(
 
 /// Delete all sync operations for a user (used during logout)
 #[tauri::command]
+#[specta::specta]
 pub async fn sync_clear_user(
     db: State<'_, DatabaseWrapper>,
     user_id: String,