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>

frontend/src/lib/api.ts

@@ -300,6 +300,7 @@ export const api = {
     customWebhookUrl?: string;
     notifyBeforeMinutes?: number;
     notificationSettings?: NotificationSettings;
+    doneRetentionDays?: number;
   }): Promise<User> {
     return request<User>('/api/v1/auth/settings', {
       method: 'PATCH',
@@ -472,6 +473,11 @@ export const api = {
     await request(`/api/v1/thoughts/${thoughtId}`, { method: 'DELETE' });
   },
 
+  // "Erledigte leeren": completed tasks of one task list ('' = all the user may edit)
+  async clearDoneTasks(taskListId: string): Promise<{ deleted: number }> {
+    return request<{ deleted: number }>('/api/v1/tasks/clear-done', { method: 'POST', body: JSON.stringify({ taskListId }) });
+  },
+
   async purgeDiscarded(): Promise<{ purged: number }> {
     return request<{ purged: number }>('/api/v1/thoughts/discarded/purge', { method: 'DELETE' });
   },