feat: a release catalogue from MusicBrainz; request search asks it directly (M502 #5519)

Minstrel keeps its own record of what artists have released, read from
MusicBrainz, so the wanted list, search and import no longer need Lidarr.

- internal/musicbrainz: searches (artists, release groups, recordings) and
  browses (an artist's release groups; a group's releases with track lists).
  One process-wide pacer at 1.1s, shared with tag enrichment, because
  MusicBrainz's limit is per address. A 503 throttle is retried.
- Migration 0080: mb_artists, mb_release_groups, mb_artist_release_groups
  (a collaboration belongs to each credited artist), mb_releases,
  mb_release_tracks.
- internal/catalogue: Discography and Releases serve the held copy and
  re-read it when older than 7 or 30 days. An unreachable MusicBrainz
  serves the stale copy, marked. An hourly pass re-reads due artists.
- Art for what isn't held comes through /api/catalogue/art from the
  enabled cover-art sources (Cover Art Archive by release group, then the
  name-based sources) and is kept on disk, so the browser never fetches a
  third-party host. A miss is a 404 and the card shows its icon.
- Request search moves from /api/lidarr/search to /api/catalogue/search,
  with the same response shape. Albums are release groups, and "in
  library" matches albums.release_group_mbid. A 10-minute cache absorbs
  search-as-you-type. Discover suggestions point at the art endpoint
  instead of looking up Lidarr.
- Android calls the new path and loads Discover art through ServerImage.
  Installed builds older than this one lose Discover search until they
  update (no compatibility path, rule 22).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/coverart/provider.go

@@ -82,6 +82,14 @@ type AlbumCoverProvider interface {
 	FetchAlbumCover(ctx context.Context, ref AlbumRef) ([]byte, error)
 }
 
+// ReleaseGroupCoverProvider is an opt-in capability for sources that can
+// answer by MusicBrainz release group, which is how the release catalogue
+// names an album before any edition of it is held (M502 #5519).
+type ReleaseGroupCoverProvider interface {
+	AlbumCoverProvider
+	FetchReleaseGroupCover(ctx context.Context, groupMBID string) ([]byte, error)
+}
+
 // ArtistArtProvider is an opt-in capability: providers implementing
 // it can fetch artist images. Returns thumb + fanart atomically at
 // the JSON metadata step (one round-trip yields both URLs); the two

internal/coverart/provider_mbcaa.go

@@ -92,10 +92,22 @@ func (p *mbcaaProvider) Configure(s ProviderSettings) error {
 // release MBID. MBCAA is MBID-only; returns ErrNotFound when
 // ref.MBID is empty. Returns ErrNotFound on 404 or ErrTransient otherwise.
 func (p *mbcaaProvider) FetchAlbumCover(ctx context.Context, ref AlbumRef) ([]byte, error) {
+	return p.fetchFront(ctx, "release", ref.MBID)
+}
+
+// FetchReleaseGroupCover retrieves the 500px front cover the Cover Art
+// Archive chose for a release group: what a search result or a wanted album
+// shows before any edition of it is held (M502 #5519).
+func (p *mbcaaProvider) FetchReleaseGroupCover(ctx context.Context, groupMBID string) ([]byte, error) {
+	return p.fetchFront(ctx, "release-group", groupMBID)
+}
+
+// fetchFront fetches /{entity}/{mbid}/front-500, honouring the rate limit.
+func (p *mbcaaProvider) fetchFront(ctx context.Context, entity, mbid string) ([]byte, error) {
 	if !p.enabled.Load() {
 		return nil, ErrNotFound // defensive; enricher already filters on enabled
 	}
-	if ref.MBID == "" {
+	if mbid == "" {
 		return nil, ErrNotFound // MBCAA is MBID-only
 	}
 
@@ -119,7 +131,7 @@ func (p *mbcaaProvider) FetchAlbumCover(ctx context.Context, ref AlbumRef) ([]by
 	p.lastCall = time.Now()
 	p.mu.Unlock()
 
-	url := fmt.Sprintf("%s/release/%s/front-500", cfg.BaseURL, ref.MBID)
+	url := fmt.Sprintf("%s/%s/%s/front-500", cfg.BaseURL, entity, mbid)
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {
 		return nil, fmt.Errorf("coverart: build request: %w", err)
@@ -157,3 +169,5 @@ func (p *mbcaaProvider) TestConnection(ctx context.Context) error {
 	}
 	return err
 }
+
+var _ ReleaseGroupCoverProvider = (*mbcaaProvider)(nil)

internal/coverart/provider_mbcaa_test.go

@@ -166,3 +166,24 @@ func TestNewMBCAAProviderFromConfig_ReplacesRegisteredConfig(t *testing.T) {
 		t.Errorf("cfg.UserAgent = %q, want %q", gotUA, "Minstrel/test-replaced (x)")
 	}
 }
+
+func TestMBCAAProvider_FetchReleaseGroupCover_AsksByGroup(t *testing.T) {
+	var path string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		path = r.URL.Path
+		_, _ = w.Write([]byte("jpeg"))
+	}))
+	defer srv.Close()
+	p := newMBCAAProviderForTest(t, srv.URL)
+
+	body, err := p.FetchReleaseGroupCover(context.Background(), "rg-1")
+	if err != nil || string(body) != "jpeg" {
+		t.Fatalf("body %q, err %v", body, err)
+	}
+	if path != "/release-group/rg-1/front-500" {
+		t.Errorf("path = %q", path)
+	}
+	if _, err := p.FetchReleaseGroupCover(context.Background(), ""); !errors.Is(err, ErrNotFound) {
+		t.Errorf("no id: err = %v, want ErrNotFound", err)
+	}
+}