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

README.md

@@ -101,7 +101,7 @@ See [docs/Specification.md](docs/Specification.md) for the full configuration re
 | **Navidrome Client** | Subsonic API v1.16.1 communication (token-based auth) |
 | **MusicBrainz Provider** | Discography fetching with rate limiting and caching |
 | **Scanner Engine** | String normalization and fuzzy comparison |
-| **Database Layer** | SQLite persistence for settings, cache, and state |
+| **Database Layer** | SQLite persistence: `artist_settings`, `local_albums` (Navidrome sync), `external_releases` (MusicBrainz cache), `notifications_sent` |
 | **Notifier** | Scheduled Telegram notifications |
 | **Web UI** | Dashboard for browsing and managing missing releases |
 
@@ -208,7 +208,7 @@ scanner:
 | **Navidrome Client** | Взаимодействие с Subsonic API v1.16.1 (токенная аутентификация) |
 | **MusicBrainz Provider** | Загрузка дискографий с кэшированием и rate limiting |
 | **Scanner Engine** | Нормализация строк и нечёткое сравнение |
-| **Database Layer** | Хранение настроек, кэша и состояния в SQLite |
+| **Database Layer** | SQLite: `artist_settings`, `local_albums` (синхронизация из Navidrome), `external_releases` (кэш MusicBrainz), `notifications_sent` |
 | **Notifier** | Планировщик уведомлений в Telegram |
 | **Web UI** | Панель управления отсутствющими релизами |