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/server.go

@@ -34,6 +34,7 @@ type Server struct {
 	characters *backend.CharacterService
 	releases   *backend.ReleaseService
 	stats      *backend.StatisticsService
+	uploads    *backend.UploadService
 	search     *backend.SearchCache
 	// covers is nil when COVERS_PATH is not set.
 	covers *backend.CoverService
@@ -64,6 +65,7 @@ func New(cfg config.Config, q repository.Querier, database Database) (*Server, e
 	}
 	s.search = backend.NewSearchCache(s.music.SearchSoundtracks)
 	s.sync = backend.NewSyncService(q, cfg.MusicPath, s.afterSync)
+	s.uploads = backend.NewUploadService(q, cfg.MusicPath, s.sync, s.library, cfg.UploadMaxFileSize, cfg.UploadMaxTotalSize)
 
 	if cfg.CoversPath != "" {
 		s.covers, err = backend.NewCoverService(q, cfg.CoversPath, backend.CoverOptions{
@@ -167,6 +169,7 @@ func NewServerInstance() *Server {
 		logger.Fatal("Failed to create server", zap.Error(err))
 	}
 	prometheus.MustRegister(metrics.NewPoolCollector(database.Pool))
+	s.uploads.RemoveOld()
 	if err := s.sync.UpdateLibraryMetrics(ctx); err != nil {
 		logger.Error("Failed to count the library", zap.Error(err))
 	}