feat(adminalert): operator email on new feedback / word complaints

New adminalert worker polls for feedback + word complaints arriving since the last check
and coalesces a burst into one digest email per interval (5 min), inert unless a distinct
admin sender (BACKEND_SMTP_ADMIN_FROM) and recipient (BACKEND_ADMIN_EMAIL, comma-separated
allowed) are configured. The mailer gains a per-message From override and splits a
comma-separated To into separate recipients (go-mail needs them as a list). Feedback/game
stores gain CountSince/CountComplaintsSince. Unit tests cover the digest, the skip-when-
empty, and the recipient split.

backend/internal/feedback/service.go

@@ -195,6 +195,11 @@ func (svc *Service) CountUnread(ctx context.Context) (int, error) {
 	return svc.store.CountUnread(ctx)
 }
 
+// CountSince counts feedback created after since, for the operator alert worker.
+func (svc *Service) CountSince(ctx context.Context, since time.Time) (int, error) {
+	return svc.store.CountSince(ctx, since)
+}
+
 // Attachment returns a message's file name and bytes, reporting false when absent.
 func (svc *Service) Attachment(ctx context.Context, id uuid.UUID) (string, []byte, bool, error) {
 	return svc.store.Attachment(ctx, id)