feat(tasks): completed tasks are deleted after a delay (default 30 days), "Erledigte leeren"

thoughts.completed_at is set by triggers on every way a task is completed (migration 14; tasks
completed earlier count from the update). Users choose the delay (7/30/90/365 days or never,
users.done_retention_days); for tasks in a list the owner's delay counts. An hourly job deletes
expired completed tasks (not in the demo) and tells their viewers. POST /tasks/clear-done clears
completed tasks the user may edit. The done tab shows the setting and the button. v0.35.0.

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

internal/api/handlers_auth.go

@@ -403,6 +403,7 @@ func (s *Server) handleMe(w http.ResponseWriter, r *http.Request) {
 	stats, _ := s.store.GetDashboardStats(userID)
 	ns := s.store.NotificationSettingsOf(userID)
 	user.NotificationSettings = &ns
+	user.DoneRetentionDays = s.store.DoneRetentionDays(userID)
 	writeJSON(w, http.StatusOK, map[string]interface{}{
 		"user":  user,
 		"stats": stats,
@@ -512,6 +513,8 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 		CurrentPassword string `json:"currentPassword"`
 		// Which notifications go where (notify.go)
 		NotificationSettings *domain.NotificationSettings `json:"notificationSettings"`
+		// Days after which completed tasks are deleted (done_cleanup.go)
+		DoneRetentionDays *int `json:"doneRetentionDays"`
 	}
 	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
 		writeError(w, http.StatusBadRequest, "Ungültiges JSON")
@@ -598,8 +601,15 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 			return
 		}
 	}
+	if req.DoneRetentionDays != nil {
+		if err := s.store.SetDoneRetentionDays(userID, *req.DoneRetentionDays); err != nil {
+			writeError(w, http.StatusBadRequest, err.Error())
+			return
+		}
+	}
 	ns := s.store.NotificationSettingsOf(userID)
 	u.NotificationSettings = &ns
+	u.DoneRetentionDays = s.store.DoneRetentionDays(userID)
 	writeJSON(w, http.StatusOK, u)
 }