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/server.go

@@ -71,6 +71,7 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 		go srv.startAlarmTicker() // the demo sends nothing (FLAW-33)
 		go srv.startNotificationJobs()
 		go srv.startBackupScheduler()
+		go srv.startDoneCleanupTicker()
 	}
 	return srv
 }
@@ -190,6 +191,7 @@ func (s *Server) Router() http.Handler {
 		r.Get("/api/v1/tasks", s.handleGetTasks)
 		r.Post("/api/v1/tasks/{id}/unlink", s.handleUnlinkEnhancedTask)
 		r.Post("/api/v1/tasks/reorder", s.handleReorderTasks)
+		r.Post("/api/v1/tasks/clear-done", s.handleClearDoneTasks)
 		r.Post("/api/v1/task-lists/{id}/tasks/reorder", s.handleReorderTasks)
 
 		// Task Lists & Shares