feat: 支持先绑定设备后关联儿童档案

- 绑定模型新增 owner_user_id，支持 child_id 为空\n- 新增绑定后设置儿童接口 /bindings/{device_id}/child\n- 前端绑定页改为先绑设备后补充儿童\n- 补充数据库迁移脚本与 DAO 测试用例

mini-program/app/service/binding.py

@@ -9,7 +9,7 @@ class BindingService:
     def __init__(self, db):
         self.dao = BindingDAO(db)
 
-    def start_bind(self, user_id: int, device_id: str, child_id: int) -> tuple[str, datetime]:
+    def start_bind(self, user_id: int, device_id: str, child_id: int | None = None) -> tuple[str, datetime]:
         bind_token = self.dao.start_bind(user_id, device_id, child_id)
         return bind_token, datetime.utcnow()
 
@@ -31,10 +31,16 @@ class BindingService:
     def get_current_binding(self, user_id: int) -> Optional[Mapping]:
         return self.dao.get_current_by_user(user_id)
 
-    def direct_bind(self, device_id: str, child_id: int, user_id: int) -> Mapping:
+    def direct_bind(self, device_id: str, child_id: int | None, user_id: int) -> Mapping:
         self.dao.direct_bind(device_id, child_id, user_id)
         return {"device_id": device_id, "child_id": child_id}
 
+    def set_binding_child(self, device_id: str, child_id: int, user_id: int) -> Mapping:
+        ok = self.dao.set_binding_child(device_id=device_id, child_id=child_id, user_id=user_id)
+        if not ok:
+            raise ValueError("binding not found")
+        return {"device_id": device_id, "child_id": child_id}
+
     def unbind(self, device_id: str, user_id: int) -> bool:
         return self.dao.unbind(device_id, user_id)
 
@@ -42,4 +48,4 @@ class BindingService:
         rows = self.dao.list_history(device_id, limit, cursor)
         has_more = len(rows) > limit
         rows = rows[:limit]
-        return rows, has_more
+        return rows, has_more