🕐 修复系统日志时区问题

- 日志写入时使用 datetime('now', 'localtime') 替代 CURRENT_TIMESTAMP
- 日志统计和清理查询也使用本地时间
- 解决日志时间比实际时间慢8小时的问题（UTC → 北京时间）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/database.js

@@ -682,8 +682,8 @@ const SystemLogDB = {
   log({ level = 'info', category, action, message, userId = null, username = null, ipAddress = null, userAgent = null, details = null }) {
     try {
       const stmt = db.prepare(`
-        INSERT INTO system_logs (level, category, action, message, user_id, username, ip_address, user_agent, details)
-        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
+        INSERT INTO system_logs (level, category, action, message, user_id, username, ip_address, user_agent, details, created_at)
+        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, datetime('now', 'localtime'))
       `);
 
       const detailsStr = details ? (typeof details === 'string' ? details : JSON.stringify(details)) : null;
@@ -776,7 +776,7 @@ const SystemLogDB = {
     return db.prepare(`
       SELECT DATE(created_at) as date, COUNT(*) as count
       FROM system_logs
-      WHERE created_at >= datetime('now', '-' || ? || ' days')
+      WHERE created_at >= datetime('now', 'localtime', '-' || ? || ' days')
       GROUP BY DATE(created_at)
       ORDER BY date DESC
     `).all(days);
@@ -786,7 +786,7 @@ const SystemLogDB = {
   cleanup(keepDays = 90) {
     const result = db.prepare(`
       DELETE FROM system_logs
-      WHERE created_at < datetime('now', '-' || ? || ' days')
+      WHERE created_at < datetime('now', 'localtime', '-' || ? || ' days')
     `).run(keepDays);
 
     return result.changes;