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/helpers_test.go

@@ -70,6 +70,8 @@ func newTestEnv(t *testing.T, configure ...func(*config.Config)) *testEnv {
 		FrontendPath:       filepath.Join(base, "frontend"),
 		DisableSessionAuth: true,
 		ReleasesURL:        "http://127.0.0.1:1",
+		UploadMaxFileSize:  1 << 20,
+		UploadMaxTotalSize: 3 << 20,
 	}
 	for _, c := range configure {
 		c(&cfg)