fix: reset subtasks for recurring tasks

tests/test_mvp_backend.py

@@ -134,6 +134,47 @@ def test_completing_repeating_task_advances_due_date_instead_of_closing_it(clien
     assert recurrence["starts_at"].replace("Z", "") == "2026-09-08T09:00:00"
 
 
+def test_completing_repeating_task_resets_completed_subtasks_for_next_occurrence(client):
+    inbox = boot(client)
+    parent = client.post(
+        "/api/v1/tasks",
+        json={
+            "title": "每日清理",
+            "list_id": inbox["id"],
+            "due_at": "2026-09-07T09:00:00Z",
+            "rrule": "FREQ=DAILY",
+        },
+    ).json()
+    first = client.post(
+        "/api/v1/tasks",
+        json={"title": "清理下载", "list_id": inbox["id"], "parent_id": parent["id"]},
+    ).json()
+    second = client.post(
+        "/api/v1/tasks",
+        json={"title": "清理缓存", "list_id": inbox["id"], "parent_id": parent["id"]},
+    ).json()
+    for child in (first, second):
+        response = client.patch(
+            f"/api/v1/tasks/{child['id']}",
+            json={"completed": True, "version": child["version"]},
+        )
+        assert response.status_code == 200
+
+    completed = client.patch(
+        f"/api/v1/tasks/{parent['id']}",
+        json={"completed": True, "version": parent["version"]},
+    )
+
+    assert completed.status_code == 200
+    assert completed.json()["completed"] is False
+    assert completed.json()["due_at"].replace("Z", "") == "2026-09-08T09:00:00"
+    assert [child["completed"] for child in completed.json()["subtasks"]] == [False, False]
+    listed = client.get("/api/v1/tasks", params={"list_id": inbox["id"], "page": 1}).json()["items"]
+    repeated = next(task for task in listed if task["id"] == parent["id"])
+    assert [child["completed"] for child in repeated["subtasks"]] == [False, False]
+
+
+
 def test_recurrence_mutations_keep_exact_timestamp_validation(client):
     inbox = boot(client)
     task = client.post(