feat: add task and habit reordering

tests/test_mvp_backend.py

@@ -75,6 +75,29 @@ def test_recurrence_rejects_occurrence_after_cutoff(client):
     ).status_code == 422
 
 
+def test_habit_reorder_persists_in_lists_and_grid(client):
+    boot(client)
+    first = client.post("/api/v1/habits", json={"name": "第一个", "kind": "boolean", "schedule_type": "daily"}).json()
+    second = client.post("/api/v1/habits", json={"name": "第二个", "kind": "boolean", "schedule_type": "daily"}).json()
+
+    response = client.put("/api/v1/habits/reorder", json={"habit_ids": [second["id"], first["id"]]})
+    assert response.status_code == 204
+    client.post("/api/v1/habits", json={"name": "第三个", "kind": "boolean", "schedule_type": "daily"})
+    assert [habit["name"] for habit in client.get("/api/v1/habits").json()] == ["第二个", "第一个", "第三个"]
+    grid = client.get("/api/v1/habits/grid", params={"week": "2026-09-07"}).json()
+    assert [habit["name"] for habit in grid["habits"]] == ["第二个", "第一个", "第三个"]
+
+
+def test_habit_reorder_rejects_foreign_or_missing_ids(client):
+    boot(client)
+    habit = client.post("/api/v1/habits", json={"name": "自己的习惯", "kind": "boolean", "schedule_type": "daily"}).json()
+    response = client.put(
+        "/api/v1/habits/reorder",
+        json={"habit_ids": [habit["id"], "00000000-0000-0000-0000-000000000001"]},
+    )
+    assert response.status_code == 404
+
+
 def test_habit_logs_support_date_range_filter(client):
     boot(client)
     habit = client.post("/api/v1/habits", json={"name": "跑步", "kind": "boolean", "schedule_type": "daily"}).json()