Prefer UUIDs in the API (Phase 3 of the UUID migration)

- New /api/v1 endpoints: GET /soundtracks, /soundtracks/{uuid},
  /songs/{uuid} and /songs/{uuid}/file (streams the song, Range support,
  locked during sync)
- UUIDs added to every response with soundtracks or songs: queue info and
  list (SoundtrackUuid, SongUuid) and statistics (soundtrack_uuid,
  song_uuid). Existing fields stay for backward compatibility
- Play counts and broken-song removal use song UUIDs instead of
  soundtrack ID + song name, so songs with the same name are counted
  separately
- Statistics return an empty song list for soundtracks without songs
  instead of a single empty entry
- The legacy .id file cleanup stays until Phase 4 (#31)

Closes #29

internal/backend/version.go

@@ -19,8 +19,11 @@ var data = []VersionData{
 			"#21 - Refactor handlers into domain-specific files",
 			"#22 - Change VersionData Changelog from string to string array",
 			"#23 - Update all dependencies to latest versions",
+			"#29 - UUIDs in API responses, new soundtrack and song endpoints by UUID",
 			"Added search cache backend",
 			"Backend sync improvements",
+			"Faster sync that skips unchanged soundtracks",
+			"Security and stability fixes",
 		},
 	},
 	{