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

backend/server.js

@@ -128,6 +128,7 @@ function loadBusinessRoutes() {
   businessRouter.use('/polls', methodGuard(['GET','POST','PUT','DELETE']), generalLimiter, require('./routes/polls'));
   businessRouter.use('/features', methodGuard(['GET','POST','PUT','DELETE']), generalLimiter, require('./routes/features'));
   businessRouter.use('/bans', methodGuard(['GET','POST','PUT','DELETE']), generalLimiter, require('./routes/bans'));
+  businessRouter.use('/logs', methodGuard(['GET']), generalLimiter, require('./routes/logs'));
   businessRouter.use('/external', require('./routes/external'));
 
   businessRouter.get('/verify', (req, res) => res.redirect(`/#/verify?token=${req.query.token}`));
@@ -197,6 +198,7 @@ app.use((err, req, res, next) => {
   if (err.message?.includes('不支持的文件类型')) return res.status(400).json({ error: err.message });
   if (err.message?.includes('文件内容与声明类型不符')) return res.status(400).json({ error: '文件类型校验失败' });
   console.error(err);
+  try { require('./logger').logSystem('error', 'server', err.message || '服务器内部错误', { path: req.path, method: req.method, stack: String(err.stack).slice(0, 1000) }); } catch {}
   res.status(500).json({ error: '服务器内部错误' });
 });