feat(api): 升级开放 API 并支持投递回调

- 为 `/api/open/v1` 引入 Token scope、分页游标、幂等发信、发送事件与重试/取消能力。
- 新增投递事件签名回调与状态 webhook outbox，补充相关配置、迁移和测试。
- 同步更新 Web 端 API 类型、个人中心 Token 权限管理，以及中英文文档和 OpenAPI 契约。

README.md

@@ -190,6 +190,10 @@ Mail flow:
 4. **Third-party clients**: Connect with SMTP 465/587, IMAP 993, or POP3 995; in production, configure certificates that match `LANQIN_PUBLIC_HOSTNAME`.
 5. **External mailbox access**: Users can add external IMAP accounts in personal mailbox management. Local-storage mode syncs mail into the database; remote-direct mode reads from the remote server each time and does not write into local mail tables.
 
+## Open API
+
+External integrations should use the versioned `/api/open/v1` endpoints with scoped API Tokens. See the [API guide](docs/API.md) and the machine-readable [OpenAPI 3.1 contract](docs/openapi.json). Sending supports idempotency keys; final delivery events can be ingested through a signed endpoint and all status changes can be pushed through the reliable signed webhook outbox.
+
 ## Development and Verification
 
 ```bash