fix(alarms): give up after three runs without delivery (FLAW-12)

New column alarm_failures counts ticker runs in which no channel
delivered; at three the alarm is marked as done and a warning is logged
instead of retrying every minute forever. Success, a new due date or
reopening the task starts over with three attempts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/alarms.go

@@ -151,6 +151,8 @@ func (s *Server) startAlarmTicker() {
 
 			if sentAny {
 				_ = s.store.MarkTaskAlarmSent(a.TaskID)
+			} else if gaveUp, _ := s.store.RecordTaskAlarmFailure(a.TaskID); gaveUp {
+				slog.Warn("Aufgaben-Alarm nach 3 Fehlversuchen aufgegeben", "task_id", a.TaskID, "title", a.Title)
 			}
 		}
 	}