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/db/queries/soundtrack.sql

@@ -62,3 +62,6 @@ AND deleted IS NULL;
 
 -- name: GetSoundtrackUUIDById :one
 SELECT uuid FROM soundtrack WHERE id = $1;
+
+-- name: AddSoundtrackPlayedByUUID :exec
+UPDATE soundtrack SET times_played = times_played + 1, last_played = now() WHERE uuid = $1;