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/server/api.go

@@ -148,8 +148,14 @@ func apiError(err error) error {
 		return huma.Error404NotFound(err.Error())
 	case errors.Is(err, backend.ErrInvalidName), errors.Is(err, backend.ErrInvalidImage):
 		return huma.Error400BadRequest(err.Error())
-	case errors.Is(err, backend.ErrCoversDisabled):
+	case errors.Is(err, backend.ErrCoversDisabled), errors.Is(err, backend.ErrUploadsUnavailable):
 		return huma.Error503ServiceUnavailable(err.Error())
+	case errors.Is(err, backend.ErrConflict):
+		return huma.Error409Conflict(err.Error())
+	case errors.Is(err, backend.ErrSyncing):
+		return huma.Error423Locked(err.Error())
+	case errors.Is(err, backend.ErrTooLarge):
+		return huma.Error413RequestEntityTooLarge(err.Error())
 	case errors.Is(err, backend.ErrUpstream):
 		return huma.Error502BadGateway(err.Error())
 	default: