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

containers/docker-compose.yml

@@ -9,6 +9,7 @@ services:
       POSTGRES_DB: music_aggregator
     volumes:
       - postgres_data:/var/lib/postgresql/data
+      - ./init.sql:/docker-entrypoint-initdb.d/init.sql:ro
     ports:
       - "5433:5432"
     healthcheck: