Add alternative names and cover art for soundtracks

Alternative names (#32):
- PUT /api/v1/soundtracks/{uuid}/alternative-names replaces a soundtrack's
  alternative names (trimmed, deduplicated, without the soundtrack's own name).
- alternative_names is in the soundtrack and statistics responses.
- /find matches them too and shows which name matched.

Covers (#12), stored in COVERS_PATH and off when it is not set:
- GET, PUT (upload) and DELETE /api/v1/soundtracks/{uuid}/cover, and
  PUT .../cover/source to pick a SteamGridDB game or AniList anime by ID.
  Uploaded and picked covers are never replaced automatically.
- With STEAMGRIDDB_API_KEY, a background job searches SteamGridDB, then
  AniList, by name, alternative names and the name without bracketed parts,
  at startup, after a sync and after names change. Search results are ranked
  by matching words, so "Civilization IV" gets "Sid Meier's Civilization IV".
- cover_modified (Unix seconds) is in the soundtrack and statistics responses,
  so clients can cache covers.

The new columns are added to migration 000006, which is not released yet.

Closes #12, closes #32

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

internal/server/server.go

@@ -35,6 +35,11 @@ type Server struct {
 	releases   *backend.ReleaseService
 	stats      *backend.StatisticsService
 	search     *backend.SearchCache
+	// covers is nil when COVERS_PATH is not set.
+	covers *backend.CoverService
+	// stopCovers ends the background cover search; coversDone is closed when it has ended.
+	stopCovers context.CancelFunc
+	coversDone chan struct{}
 	httpServer *http.Server
 	// metrics serves /metrics on its own port; nil when metrics are off.
 	metrics *metrics.Server
@@ -57,9 +62,19 @@ func New(cfg config.Config, q repository.Querier, database Database) (*Server, e
 		releases:   backend.NewReleaseService(cfg.ReleasesURL),
 		stats:      backend.NewStatisticsService(q),
 	}
-	s.search = backend.NewSearchCache(s.music.SoundtrackNames)
+	s.search = backend.NewSearchCache(s.music.SearchSoundtracks)
 	s.sync = backend.NewSyncService(q, cfg.MusicPath, s.afterSync)
 
+	if cfg.CoversPath != "" {
+		s.covers, err = backend.NewCoverService(q, cfg.CoversPath, backend.CoverOptions{
+			SteamGridDBKey: cfg.SteamGridDBAPIKey,
+			SteamGridDBURL: cfg.SteamGridDBURL,
+			AniListURL:     cfg.AniListURL,
+		})
+		if err != nil {
+			return nil, fmt.Errorf("covers path: %w", err)
+		}
+	}
 	if cfg.MetricsPort > 0 {
 		s.metrics = metrics.NewServer(cfg.MetricsPort)
 	}
@@ -73,6 +88,36 @@ func New(cfg config.Config, q repository.Querier, database Database) (*Server, e
 	return s, nil
 }
 
+// startCoverSearch runs the background cover search until Shutdown. It does nothing when
+// covers are off or there is no SteamGridDB API key.
+func (s *Server) startCoverSearch() {
+	if s.covers == nil || !s.covers.SearchesAutomatically() {
+		logging.GetLogger().Info("Automatic cover search is off",
+			zap.Bool("covers_path_set", s.covers != nil))
+		return
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	s.stopCovers, s.coversDone = cancel, make(chan struct{})
+	go func() {
+		defer close(s.coversDone)
+		s.covers.Run(ctx)
+	}()
+}
+
+// soundtracksChanged reloads the cached soundtrack data after soundtracks were edited, and
+// lets the cover search try the new names.
+func (s *Server) soundtracksChanged(ctx context.Context) {
+	if err := s.music.ReloadSoundtracks(ctx); err != nil {
+		logging.GetLogger().Error("Failed to reload soundtracks", zap.Error(err))
+	}
+	if err := s.search.Reload(ctx); err != nil {
+		logging.GetLogger().Error("Failed to reload search cache", zap.Error(err))
+	}
+	if s.covers != nil {
+		s.covers.Trigger()
+	}
+}
+
 // afterSync refreshes cached data once a sync has finished.
 func (s *Server) afterSync(ctx context.Context) {
 	if err := s.music.EndAllQueues(ctx); err != nil {
@@ -84,6 +129,9 @@ func (s *Server) afterSync(ctx context.Context) {
 	if err := s.search.Reload(ctx); err != nil {
 		logging.GetLogger().Error("Failed to reload search cache after sync", zap.Error(err))
 	}
+	if s.covers != nil {
+		s.covers.Trigger()
+	}
 }
 
 // NewServerInstance loads the configuration, connects to and migrates the database and creates the Server.
@@ -133,6 +181,7 @@ func (s *Server) ListenAndServe() error {
 			logging.GetLogger().Error("Failed to start the metrics server", zap.Error(err))
 		}
 	}
+	s.startCoverSearch()
 	logging.GetLogger().Info("Server starting", zap.String("address", s.httpServer.Addr))
 	if err := s.httpServer.ListenAndServe(); !errors.Is(err, http.ErrServerClosed) {
 		return err
@@ -146,6 +195,10 @@ func (s *Server) Shutdown(ctx context.Context) error {
 	if s.metrics != nil {
 		err = errors.Join(err, s.metrics.Shutdown(ctx))
 	}
+	if s.stopCovers != nil {
+		s.stopCovers()
+		<-s.coversDone
+	}
 	s.sync.Wait()
 	s.characters.Close()
 	s.db.Close()