[verified] feat: support monthly completion recurrence

tests/test_backup_v2.py

@@ -200,6 +200,36 @@ def test_merge_same_backup_is_idempotent_via_import_ledger(client):
     assert asyncio.run(counts()) == (1, 1)
 
 
+def test_legacy_backup_without_completion_unit_is_idempotent(client):
+    inbox = boot(client)
+    client.post(
+        "/api/v1/tasks",
+        json={
+            "title": "旧版完成后重复",
+            "list_id": inbox["id"],
+            "due_at": "2026-03-08T01:30:00Z",
+            "trigger_mode": "after_completion",
+            "after_completion_days": 2,
+        },
+    )
+    content = client.get("/api/v1/backup/export.zip").content
+    recurrences = _archive_rows(content, "recurrences")
+    for recurrence in recurrences:
+        recurrence.pop("after_completion_unit", None)
+    legacy_content = _replace_entities(content, {"recurrences": recurrences})
+
+    first_token = _preflight(client, legacy_content).json()["preflight_token"]
+    assert client.post(
+        "/api/v1/backup/restore", json={"preflight_token": first_token, "mode": "merge"}
+    ).status_code == 200
+    second_token = _preflight(client, legacy_content).json()["preflight_token"]
+    second = client.post(
+        "/api/v1/backup/restore", json={"preflight_token": second_token, "mode": "merge"}
+    )
+    assert second.status_code == 200
+    assert second.json()["already_imported"] is True
+
+
 def test_invalid_zip_variants_are_rejected_before_any_write(client):
     inbox = boot(client)
     before = len(client.get("/api/v1/tasks", params={"list_id": inbox["id"]}).json()["items"])