fix(server,web/m7-353): post-review fixes (test compilation, AlbumRef field, bulk handler, design system)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/types.go

@@ -65,7 +65,8 @@ type AlbumRef struct {
 	Year        int    `json:"year,omitempty"`
 	TrackCount  int    `json:"track_count"`
 	DurationSec int    `json:"duration_sec"`
-	CoverURL    string `json:"cover_url"`
+	CoverURL        string  `json:"cover_url"`
+	CoverArtSource  *string `json:"cover_art_source"`
 }
 
 // TrackRef is the lightweight track shape used in album details and search.