feat: system logs - email log + system log with admin UI

- db: email_logs (sent/failed) + system_logs (info/warn/error) tables,
  auto-migrated for existing installs
- backend/logger.js: logEmail/logSystem with try/catch (never breaks flow)
- mailer: log send result (SMTP unconfigured/sent/failed + error msg)
- webhook: log blocked-internal, non-2xx response, send failure
- server: error middleware records 500 errors to system_logs
- routes/logs.js: GET /logs/emails + /logs/system (admin/owner, filters)
- UI: 系统日志 page (owner/admin) with system/email tabs + filters
- API docs updated

docs/API.md

@@ -301,6 +301,19 @@ Base URL: `http://<host>:3100/api`
 
 ---
 
+## 九·五、日志 Logs *(admin/owner)*
+
+系统自动记录邮件发送结果、Webhook 调用、服务器错误,后台「系统日志」页可查,方便排查。
+
+| 端点 | 说明 |
+|------|------|
+| GET /logs/emails?status=sent\|failed&email=&limit= | 邮件日志(收件人/模板/主题/状态/错误) |
+| GET /logs/system?level=info\|warn\|error&source=&limit= | 系统日志(级别/来源/消息/详情) |
+
+记录来源:mailer(SMTP 未配置/发送成功/失败)、webhook(拦截内网/非2xx/发送失败)、server(500 错误中间件)。日志写库失败不影响主流程,表自动迁移创建。
+
+---
+
 ## 十、验证码与文件
 
 | 端点 | 说明 |