fix(media): address review comments on TTL cleanup

- CleanExpired: split into two phases — collect expired entries under
  lock, then delete files after releasing the lock to minimize contention
- CleanExpired: guard against zero MaxAge (no-op if unconfigured)
- CleanExpired: log file removal errors instead of silently ignoring
- Start: protect with startOnce to prevent multiple goroutines
- Stop: rename once -> stopOnce for clarity
- cmd_gateway: call mediaStore.Stop() on error path after Start()
- Add TestCleanExpiredZeroMaxAge and double-Start test

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/picoclaw/internal/gateway/helpers.go

@@ -124,6 +124,7 @@ func gatewayCmd(debug bool) error {
 
 	channelManager, err := channels.NewManager(cfg, msgBus, mediaStore)
 	if err != nil {
+		mediaStore.Stop()
 		return fmt.Errorf("error creating channel manager: %w", err)
 	}