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/mailer.js

@@ -1,5 +1,6 @@
 const nodemailer = require('nodemailer');
 const { getPool, query, getRow, getConfig } = require('./db');
+const { logEmail, logSystem } = require('./logger');
 
 async function getSmtpConfig() {
   try {
@@ -56,16 +57,21 @@ function getTransporter(cfg) {
 async function sendEmail(to, templateCode, vars) {
   try {
     const cfg = await getSmtpConfig();
-    if (!cfg.host || !cfg.user) { console.log('[Mailer] SMTP未配置'); return false; }
+    if (!cfg.host || !cfg.user) { console.log('[Mailer] SMTP未配置'); await logSystem('warn', 'mailer', 'SMTP未配置，邮件未发送', { to, template: templateCode }); return false; }
     const template = await getTemplate(templateCode);
-    if (!template) { console.log('[Mailer] 模板不存在:', templateCode); return false; }
+    if (!template) { console.log('[Mailer] 模板不存在:', templateCode); await logEmail(to, templateCode, '', 'failed', '模板不存在: ' + templateCode); return false; }
     const siteName = await getSiteName();
     const allVars = { ...vars, site_name: siteName, site_url: await getSiteUrl() };
     const { subject, body } = renderTemplate(template, allVars);
     const transporter = getTransporter(cfg);
     await transporter.sendMail({ from: `"${siteName}" <${cfg.from}>`, to, subject, html: body });
+    await logEmail(to, templateCode, subject, 'sent');
     return true;
-  } catch (err) { console.error('[Mailer]', err.message); return false; }
+  } catch (err) {
+    console.error('[Mailer]', err.message);
+    await logEmail(to, templateCode, '', 'failed', err.message);
+    return false;
+  }
 }
 
 module.exports = { sendEmail, renderTemplate, getTemplate, getSmtpConfig, getSiteName, getSiteUrl };