fix(tts): fix pocket-tts voices missing in UI and 500 on first TTS enqueue

- Add POCKET_TTS_URL env to backend service in docker-compose.yml so
  pocket-tts voices appear in the voice selector (Doppler secret existed
  but the env var was never passed to the container)
- Fix GetAudioTask PocketBase filter using %q (double-quotes) instead of
  single-quoted string, causing the duplicate-task guard to always miss
- Fix AudioPlayer double-POST: GET /api/presign/audio already enqueues
  TTS internally on 404; AudioPlayer now skips the redundant POST and
  polls directly, eliminating the 500 from the PB unique-key conflict

backend/internal/storage/store.go

@@ -706,7 +706,7 @@ func (s *Store) ListAudioTasks(ctx context.Context) ([]domain.AudioTask, error)
 }
 
 func (s *Store) GetAudioTask(ctx context.Context, cacheKey string) (domain.AudioTask, bool, error) {
-	filter := fmt.Sprintf(`cache_key=%q`, cacheKey)
+	filter := fmt.Sprintf(`cache_key='%s'`, cacheKey)
 	items, err := s.pb.listAll(ctx, "audio_jobs", filter, "-started")
 	if err != nil || len(items) == 0 {
 		return domain.AudioTask{}, false, err