feat(notify): bundled notifications, task watchers, daily summary

- Per-user routing (own mail / own Discord webhook) for task alarms
  (sent at once), shares, completed shared tasks and watched tasks;
  settings as JSON on the user (GET /auth/me, PATCH /auth/settings).
- Everything but alarms is queued and sent as one message per user
  after NOTIFY_DIGEST_MINUTES (default 5); mails now multipart with an
  HTML version, Discord as one embed. Three failed attempts drop it.
- Own changes never notify the actor, except on watched tasks when
  asked for; "paused" stops everything including alarms.
- Task watchers: POST/DELETE /tasks/{id}/watch, isWatching on tasks.
- Daily summary at a chosen time: overdue, today, tomorrow, plus the
  count of the next 7 days; nothing due, nothing sent.
- Migration 7; jobs do not run in the demo.

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

internal/api/handlers_tasklists.go

@@ -125,6 +125,9 @@ func (s *Server) handleCreateTaskListShare(w http.ResponseWriter, r *http.Reques
 		return
 	}
 	s.broadcastTaskListChange(listID, nil, "TASK_LIST_UPDATED")
+	if tl, err := s.store.GetTaskListByID(userID, listID); err == nil {
+		s.notifyShared(userID, req.TargetType, req.TargetID, "die Aufgabenliste", tl.Title)
+	}
 	writeJSON(w, http.StatusCreated, share)
 }