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

Cargo.toml

@@ -25,6 +25,7 @@ roxmltree = "0.20"
 base64 = "0.22"
 chrono = { version = "0.4", features = ["serde"] }
 clap = { version = "4", features = ["derive"] }
+sqlx = { version = "0.8", features = ["runtime-tokio", "postgres", "uuid", "chrono", "json"] }
 tonic = "0.12"
 prost = "0.13"