feat: 三大核心功能完成 — query_history持久化 + EXPLAIN真实解析 + Dashboard修复

P1 查询历史持久化 (47 tests):
- QueryHistory/QuickQuery SQLAlchemy model 建表
- query_history_service 从内存迁移到 SQLite async
- 每用户500条上限自动清理
- 7个API endpoint全部对接持久化
- 10个预置快捷查询模板

P2 EXPLAIN真实解析 (40 tests):
- MySQL EXPLAIN FORMAT=JSON 执行+解析
- PostgreSQL EXPLAIN (FORMAT JSON) 执行+解析
- SQLite EXPLAIN QUERY PLAN 解析
- 统一树结构输出 + 优化警告
- POST /api/v1/sql-assistant/explain endpoint

Dashboard修复:
- 修复重复export default导致页面崩溃
- 对接/instances /tickets /dashboard/stats真实API
- 替换硬编码统计数字为API驱动
- 修复calcHealthScore安全访问

AuditLogsPage:
- 修复Fragment key warning

1081 tests passed (10 system_settings failures are pre-existing)

frontend/src/pages/AuditLogsPage.tsx

@@ -1,4 +1,4 @@
-import { useState, useEffect, useCallback, useMemo } from 'react';
+import { useState, useEffect, useCallback, useMemo, Fragment } from 'react';
 import {
   Search, Download, Filter, ChevronDown, ChevronRight,
   FileText, User, Globe, CheckCircle2, XCircle, AlertTriangle,
@@ -440,7 +440,7 @@ export default function AuditLogsPage() {
                     const StatusIcon = statusConfig.icon;
                     const TargetIcon = TARGET_ICONS[log.target_type] || FileText;
                     return (
-                      <>
+                      <Fragment key={log.id}>
                         <tr
                           key={log.id}
                           className={`border-b border-[var(--dg-border-light)] transition-colors cursor-pointer ${
@@ -516,7 +516,7 @@ export default function AuditLogsPage() {
                             </td>
                           </tr>
                         )}
-                      </>
+                      </Fragment>
                     );
                   })
                 )}