feat(gateway): 下单任务支持 callback_url 终结类事件异步通知

- POST /api/orders 新增可选 callback_url（仅 http/https，其余 422）
- 仅终结类事件各通知一次：terminal report 推入终态（succeeded/failed/
  needs_human）、租约过期被 sweep 置 stale；中间态与终结后的监控上报不通知
- 幂等重发不更新既有任务的回调地址；投递 best-effort 单次尝试，失败只记日志
- CallbackNotifier 发后不管（create_task + 在途任务强引用），关停等在途发完；
  生产路径按回调地址逐次构造客户端，满足统一出站代理策略（test_proxy.py）
- tasks 表加 callback_url 列，GatewayDB.start() 内置迁移兼容既有库
- 新增 RAKUTEN_CALLBACK_TIMEOUT_SECONDS（默认 10）；文档补 §4.8；
  openapi.json 重新导出（gitignore 未跟踪）；新增 17 条测试，全量 492 通过

app/gateway/db.py

@@ -25,6 +25,7 @@ CREATE TABLE IF NOT EXISTS tasks (
     task_id          TEXT PRIMARY KEY,
     site             TEXT NOT NULL,
     intent_json      TEXT NOT NULL,
+    callback_url     TEXT,           -- 终结类事件通知地址，见 docs/order-gateway.md §4.8
     status           TEXT NOT NULL,
     lease_owner      TEXT,
     lease_expires_at TEXT,
@@ -98,6 +99,7 @@ class TaskRow:
     task_id: str
     site: str
     intent_json: str
+    callback_url: str | None
     status: str
     lease_owner: str | None
     lease_expires_at: str | None
@@ -180,6 +182,7 @@ def _row_to_task(row: aiosqlite.Row) -> TaskRow:
         task_id=row["task_id"],
         site=row["site"],
         intent_json=row["intent_json"],
+        callback_url=row["callback_url"],
         status=row["status"],
         lease_owner=row["lease_owner"],
         lease_expires_at=row["lease_expires_at"],
@@ -256,9 +259,18 @@ class GatewayDB:
         self._conn = await aiosqlite.connect(str(self._db_path))
         self._conn.row_factory = aiosqlite.Row
         await self._conn.executescript(SCHEMA)
+        await self._migrate()
         await self._conn.commit()
         logger.info("网关 DB 已就绪：%s", self._db_path)
 
+    async def _migrate(self) -> None:
+        """给既有库补新列（CREATE TABLE IF NOT EXISTS 不会改老表）"""
+        async with self.conn.execute("PRAGMA table_info(tasks)") as cur:
+            columns = {row["name"] for row in await cur.fetchall()}
+        if "callback_url" not in columns:
+            await self.conn.execute("ALTER TABLE tasks ADD COLUMN callback_url TEXT")
+            logger.info("迁移：tasks 表补充 callback_url 列")
+
     async def close(self) -> None:
         if self._conn is not None:
             await self._conn.close()
@@ -281,13 +293,14 @@ class GatewayDB:
         """插入新任务。返回 True=新建，False=task_id 已存在（幂等命中）"""
         try:
             await self.conn.execute(
-                "INSERT INTO tasks (task_id, site, intent_json, status, "
+                "INSERT INTO tasks (task_id, site, intent_json, callback_url, status, "
                 "lease_owner, lease_expires_at, lease_count, created_at, updated_at) "
-                "VALUES (?, ?, ?, ?, NULL, NULL, 0, ?, ?)",
+                "VALUES (?, ?, ?, ?, ?, NULL, NULL, 0, ?, ?)",
                 (
                     row.task_id,
                     row.site,
                     row.intent_json,
+                    row.callback_url,
                     row.status,
                     row.created_at,
                     row.updated_at,