feat: add Navidrome client with Subsonic API sync

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

CLAUDE.md

@@ -111,9 +111,11 @@ Based on the specification (docs/Specification.md), the application follows a mo
 
 4. **Database Layer** (`internal/database/` or similar)
    - SQLite 3 integration via github.com/mattn/go-sqlite3
+   - Subsonic API client via github.com/delucks/go-subsonic
    - Manages schema migrations
    - Handles tables:
      - `artist_settings`: Artist monitoring configuration
+     - `local_albums`: Navidrome albums synced via Subsonic API (id, artist_id, title)
      - `external_releases`: Cached MusicBrainz data
      - `notifications_sent`: Sent notification tracking