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/testutil/fakedb/fakedb.go

@@ -180,6 +180,17 @@ func (s *Store) AddSoundtrackPlayedByUUID(_ context.Context, id uuid.UUID) error
 	return s.Err
 }
 
+func (s *Store) GetSoundtrackById(_ context.Context, id int32) (repository.Soundtrack, error) {
+	defer s.lock()()
+	if s.Err != nil {
+		return repository.Soundtrack{}, s.Err
+	}
+	if i := s.soundtrackIndex(id); i >= 0 && s.Soundtracks[i].Deleted == nil {
+		return s.Soundtracks[i], nil
+	}
+	return repository.Soundtrack{}, pgx.ErrNoRows
+}
+
 func (s *Store) GetSoundtrackByUUID(_ context.Context, id uuid.UUID) (repository.Soundtrack, error) {
 	defer s.lock()()
 	if s.Err != nil {