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/errors.go

@@ -15,6 +15,14 @@ var (
 	ErrInvalidImage = errors.New("invalid image")
 	// ErrCoversDisabled is returned when covers are used but COVERS_PATH is not set.
 	ErrCoversDisabled = errors.New("covers are not enabled on this server")
+	// ErrConflict is returned when something with the same name already exists.
+	ErrConflict = errors.New("already exists")
+	// ErrSyncing is returned when the music folder is being synced.
+	ErrSyncing = errors.New("syncing is in progress")
+	// ErrTooLarge is returned when an upload is over its size limit.
+	ErrTooLarge = errors.New("too large")
+	// ErrUploadsUnavailable is returned when the music folder cannot be written to.
+	ErrUploadsUnavailable = errors.New("uploads are not possible: the music folder cannot be written to")
 	// ErrUpstream is returned when an external service could not be reached.
 	ErrUpstream = errors.New("upstream service unavailable")
 )