fix: auto-scan cloud libraries on boot + allow CORS for media playback

- Add BootCloudLibraries() to auto-scan all cloud libraries on startup
- Delay 3s to avoid conflict with system init, scan without auto-scrape
- Enable CORS for /api/cloud/play/* and /api/img to support 3rd-party players
- Fixes issue where each user triggers separate cloud library scans
- Fixes issue where Infuse/Emby apps cannot play cloud resources

internal/service/transmission_adp.go

@@ -84,7 +84,7 @@ func (a *TransmissionAdapter) pingLocked(ctx context.Context) error {
 		return err
 	}
 	defer resp.Body.Close()
-	io.Copy(io.Discard, resp.Body)
+	_, _ = io.Copy(io.Discard, resp.Body)
 	if resp.StatusCode == 409 {
 		// 正常：需要 CSRF token
 		a.sessionID = resp.Header.Get("X-Transmission-Session-Id")