Add the Navidrome client module that connects to a Navidrome server via
the Subsonic API, fetches artist and album data, and syncs it into the
local SQLite database.
- Add go-subsonic dependency for Subsonic API communication
- Create internal/navidrome/client.go with NavidromeClient wrapper
- NewClient constructor with token-based auth
- Ping health check with HTTP status validation
- GetArtists fetches all artists via getArtists endpoint
- GetArtistAlbums fetches albums per artist via getArtist endpoint
- Create internal/navidrome/sync.go with sync orchestration
- SyncArtists upserts artists into artist_settings table
- SyncAlbums fetches and stores albums for monitored artists
- Add local_albums table (migration 003) with FK to artist_settings
- Add LocalAlbum CRUD operations in internal/database/local_albums.go
- Full test coverage: 19 tests across client and sync packages
- All tests pass, go vet and go fmt clean
