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

public/index.html

@@ -74,6 +74,7 @@
   <script src="js/pages/polls.js"></script>
   <script src="js/pages/servers.js"></script>
   <script src="js/pages/bans.js"></script>
+  <script src="js/pages/logs.js"></script>
   <script src="js/pages/features.js"></script>
   <script src="js/pages/feature-detail.js"></script>
   <script src="js/app.js"></script>