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/game/service.go

@@ -1008,6 +1008,12 @@ func (svc *Service) CountComplaints(ctx context.Context, status string) (int, er
 	return svc.store.CountComplaints(ctx, status)
 }
 
+// CountComplaintsSince counts word complaints filed after since, for the operator alert
+// worker.
+func (svc *Service) CountComplaintsSince(ctx context.Context, since time.Time) (int, error) {
+	return svc.store.CountComplaintsSince(ctx, since)
+}
+
 // ResolveComplaint closes a complaint with an operator disposition (reject /
 // accept_add / accept_remove) and an optional note. An accepted complaint then
 // appears in DictionaryChanges until a rebuilt dictionary is loaded and the