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/logger.js

@@ -0,0 +1,28 @@
+// 日志模块: 邮件日志 + 系统日志, 写入数据库供后台查看
+// 所有写库操作 try/catch 包裹, 日志失败绝不影响主流程
+const { query } = require('./db');
+
+// 系统日志: level = info | warn | error
+async function logSystem(level, source, message, details) {
+  try {
+    await query(
+      'INSERT INTO system_logs(level, source, message, details) VALUES (?,?,?,?)',
+      [level || 'info', String(source || '').slice(0, 50), String(message || '').slice(0, 2000),
+       details ? JSON.stringify(details).slice(0, 4000) : null]
+    );
+  } catch { /* 日志失败忽略 */ }
+}
+
+// 邮件日志: status = sent | failed
+async function logEmail(to, templateCode, subject, status, error) {
+  try {
+    await query(
+      'INSERT INTO email_logs(to_email, template_code, subject, status, error) VALUES (?,?,?,?,?)',
+      [String(to || '').slice(0, 100), String(templateCode || '').slice(0, 50),
+       String(subject || '').slice(0, 255), status === 'sent' ? 'sent' : 'failed',
+       error ? String(error).slice(0, 2000) : null]
+    );
+  } catch { /* 日志失败忽略 */ }
+}
+
+module.exports = { logSystem, logEmail };