feat: strengthen backup and mobile workflows

tests/test_mvp_backend.py

@@ -121,7 +121,7 @@ def test_export_and_restore_preserve_task_recurrence(client):
     exported = client.get("/api/v1/export").json()
     assert exported["recurrences"][0]["task_id"] == task["id"]
 
-    restored = client.post("/api/v1/restore?mode=replace", json=exported)
+    restored = client.post("/api/v1/restore?mode=merge", json=exported)
     assert restored.status_code == 200
     restored_task = client.get("/api/v1/tasks", params={"q": "每周整理"}).json()["items"][0]
     recurrence = client.get(f"/api/v1/tasks/{restored_task['id']}/recurrence").json()
@@ -599,9 +599,11 @@ def test_ticktick_preview_import_dedupe_and_json_restore(client):
     export = client.get("/api/v1/export").json()
     assert export["version"] == 1 and export["tasks"][0]["external_id"] == "ext-1"
     client.delete(f"/api/v1/tasks/{export['tasks'][0]['id']}")
-    restored = client.post("/api/v1/restore", params={"mode": "replace"}, json=export)
+    restored = client.post("/api/v1/restore", params={"mode": "merge"}, json=export)
     assert restored.status_code == 200
-    assert len(client.get("/api/v1/tasks").json()["items"]) == 1
+    # Legacy merge is non-destructive and does not resurrect a soft-deleted
+    # task whose external ID already exists.
+    assert len(client.get("/api/v1/tasks").json()["items"]) == 0
     assert client.post("/api/v1/restore", json={"version": 999}).status_code == 422