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_auth.go

@@ -341,6 +341,8 @@ func (s *Server) handleMe(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	stats, _ := s.store.GetDashboardStats(userID)
+	ns := s.store.NotificationSettingsOf(userID)
+	user.NotificationSettings = &ns
 	writeJSON(w, http.StatusOK, map[string]interface{}{
 		"user":  user,
 		"stats": stats,
@@ -448,6 +450,8 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 		NotifyBeforeMinutes *int      `json:"notifyBeforeMinutes"`
 		// Needed to change password or e-mail address (FLAW-30)
 		CurrentPassword string `json:"currentPassword"`
+		// Which notifications go where (notify.go)
+		NotificationSettings *domain.NotificationSettings `json:"notificationSettings"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, http.StatusBadRequest, "Ungültiges JSON")
@@ -528,6 +532,14 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 	if changesPassword || changesEmail {
 		s.notifyAccountChange(before, changesPassword, changesEmail, u.Email)
 	}
+	if req.NotificationSettings != nil {
+		if err := s.store.SetNotificationSettings(userID, *req.NotificationSettings); err != nil {
+			writeError(w, http.StatusInternalServerError, "Benachrichtigungen konnten nicht gespeichert werden")
+			return
+		}
+	}
+	ns := s.store.NotificationSettingsOf(userID)
+	u.NotificationSettings = &ns
 	writeJSON(w, http.StatusOK, u)
 }