feat: add PostgreSQL persistence and unified sync endpoint

- Add sqlx with PostgreSQL support for database operations
- Create DbService with artist/album upsert and listing methods
- Add database schema (14 tables) in containers/init.sql
- Add library controller (GET /api/library/artists, /albums, /stats)
- Merge sync_artist + ingest into single POST /api/sync endpoint
- Support configurable sync: download (bool), store (bool), album filter
- Connect to database at startup with graceful fallback

src/api/mod.rs

@@ -1,4 +1,5 @@
 mod indexer_controller;
+mod library_controller;
 mod metadata_controller;
 mod sync_controller;
 mod torrent_controller;
@@ -29,6 +30,7 @@ pub fn routes(state: AppState) -> Router {
         .nest("/torrents", torrent_controller::routes())
         .nest("/metadata", metadata_controller::routes())
         .nest("/sync", sync_controller::routes())
+        .nest("/library", library_controller::routes())
         .with_state(state)
 }