Upload new soundtracks through the API

A client starts an upload with the soundtrack's name, sends one MP3 per
request, and finishes it. The server then moves the songs into a new folder
in the music folder and syncs only that folder, so the soundtrack is in the
library, lists and search right away.

- POST /api/v1/soundtracks/uploads, GET/DELETE .../uploads/{id},
  PUT .../uploads/{id}/files/{file} and POST .../uploads/{id}/finish.
- Names must work as file names on every system and be free, ignoring case.
  File names are plain: clients flatten disc folders.
- Only MP3 files. Size limits from UPLOAD_MAX_FILE_MB (50) and
  UPLOAD_MAX_TOTAL_MB (2048).
- Refused while a sync runs, and a sync cannot start while an upload is
  finished. Songs equal to an existing soundtrack's are refused, as the sync
  would take them for that soundtrack renamed.
- Unfinished uploads live in MUSIC_PATH/.uploads and are removed after a day.
  The sync now skips every hidden folder.

Closes #33

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

internal/backend/sync.go

@@ -35,8 +35,15 @@ const (
 	brokenWorkers     = 50
 )
 
-// foldersToSkip are directories in the music path that are never synced.
-var foldersToSkip = map[string]struct{}{".sync": {}, "characters": {}, "dist": {}, "old": {}}
+// foldersToSkip are directories in the music path that are never synced, besides hidden
+// ones like .sync and .uploads.
+var foldersToSkip = map[string]struct{}{"characters": {}, "dist": {}, "old": {}}
+
+// skipFolder reports whether a directory in the music path is not a soundtrack.
+func skipFolder(name string) bool {
+	_, skip := foldersToSkip[name]
+	return skip || strings.HasPrefix(name, ".")
+}
 
 type SyncResponse struct {
 	SoundtracksAdded          []string          `json:"soundtracks_added"`
@@ -117,6 +124,37 @@ func (s *SyncService) Start(full bool) bool {
 	return true
 }
 
+// SyncFolder runs prepare and then syncs the soundtrack folder name, while no other sync
+// can run. It returns ErrSyncing when a sync is running. prepare can put the folder in
+// place; when it fails, nothing is synced. onComplete is not called, as only one
+// soundtrack changes.
+func (s *SyncService) SyncFolder(ctx context.Context, name string, prepare func() error) (repository.Soundtrack, error) {
+	if !s.running.CompareAndSwap(false, true) {
+		return repository.Soundtrack{}, ErrSyncing
+	}
+	defer s.running.Store(false)
+
+	existing, err := s.q.GetAllSoundtracksIncludingDeleted(ctx)
+	if err != nil {
+		return repository.Soundtrack{}, err
+	}
+	if err := prepare(); err != nil {
+		return repository.Soundtrack{}, err
+	}
+	run := &syncRun{q: s.q, musicPath: s.musicPath, existing: existing, progress: func() {}}
+	run.syncSoundtrack(ctx, name)
+	if len(run.errs) > 0 {
+		return repository.Soundtrack{}, fmt.Errorf("syncing %q: %s", name, strings.Join(run.errs, "; "))
+	}
+	if len(run.foundIDs) != 1 {
+		return repository.Soundtrack{}, fmt.Errorf("syncing %q found %d soundtracks", name, len(run.foundIDs))
+	}
+	if err := s.UpdateLibraryMetrics(ctx); err != nil {
+		logging.GetLogger().Error("Failed to count the library", zap.Error(err))
+	}
+	return s.q.GetSoundtrackById(ctx, run.foundIDs[0])
+}
+
 // Wait blocks until the running sync, if any, has finished.
 func (s *SyncService) Wait() {
 	s.wg.Wait()
@@ -276,7 +314,7 @@ func (r *syncRun) execute(ctx context.Context, setTotal func(int)) SyncResponse
 	for _, dir := range directories {
 		g.Go(func() error {
 			defer r.progress()
-			if _, skip := foldersToSkip[dir.Name()]; dir.IsDir() && !skip {
+			if dir.IsDir() && !skipFolder(dir.Name()) {
 				r.syncSoundtrack(ctx, dir.Name())
 			}
 			return nil