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

internal/database/database.go

@@ -57,6 +57,11 @@ func (db *DB) Conn() *sql.DB {
 	return db.conn
 }
 
+// Begin starts a new database transaction.
+func (db *DB) Begin() (*sql.Tx, error) {
+	return db.conn.Begin()
+}
+
 // migrate runs all pending schema migrations in order.
 func (db *DB) migrate() error {
 	// Create the migrations tracking table first, unconditionally.
@@ -94,7 +99,16 @@ func (db *DB) migrate() error {
 			);`,
 		},
 		{
-			name: "003_create_notifications_sent",
+			name: "003_create_local_albums",
+			sql: `CREATE TABLE IF NOT EXISTS local_albums (
+				id TEXT PRIMARY KEY,
+				artist_id TEXT NOT NULL REFERENCES artist_settings(id),
+				title TEXT NOT NULL
+			);
+			CREATE INDEX IF NOT EXISTS idx_local_albums_artist_id ON local_albums(artist_id);`,
+		},
+		{
+			name: "004_create_notifications_sent",
 			sql: `CREATE TABLE IF NOT EXISTS notifications_sent (
 				rgid TEXT NOT NULL REFERENCES external_releases(rgid),
 				sent_at DATETIME DEFAULT CURRENT_TIMESTAMP,
@@ -145,7 +159,6 @@ func (db *DB) isMigrationApplied(name string) (bool, error) {
 	return count > 0, nil
 }
 
-
 // ArtistSettings represents a row in the artist_settings table.
 type ArtistSettings struct {
 	ID                 string `json:"id"`
@@ -155,6 +168,13 @@ type ArtistSettings struct {
 	Monitored          bool   `json:"monitored"`
 }
 
+// LocalAlbum represents a row in the local_albums table.
+type LocalAlbum struct {
+	ID       string `json:"id"`
+	ArtistID string `json:"artist_id"`
+	Title    string `json:"title"`
+}
+
 // ExternalRelease represents a row in the external_releases table.
 type ExternalRelease struct {
 	RGID        string `json:"rgid"`