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)

backend/app/api/v1/endpoints/query_history.py

@@ -18,6 +18,7 @@ async def my_query_history(
     keyword: Optional[str] = None,
     page: int = 1,
     page_size: int = 20,
+    db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
     """获取我的查询历史。
@@ -25,6 +26,7 @@ async def my_query_history(
     自动记录每次查询，支持按数据库类型、实例、关键词筛选。
     """
     return await QueryHistoryService.get_history(
+        db=db,
         user_id=current_user.id,
         db_type=db_type,
         instance_id=instance_id,
@@ -37,10 +39,11 @@ async def my_query_history(
 @router.post("/{history_id}/favorite")
 async def toggle_favorite(
     history_id: int,
+    db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
     """切换查询历史收藏状态。"""
-    is_fav = await QueryHistoryService.toggle_favorite(current_user.id, history_id)
+    is_fav = await QueryHistoryService.toggle_favorite(db, current_user.id, history_id)
     return {"is_favorite": is_fav, "message": "已收藏" if is_fav else "已取消收藏"}
 
 
@@ -48,20 +51,22 @@ async def toggle_favorite(
 async def my_favorites(
     page: int = 1,
     page_size: int = 20,
+    db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
     """获取我收藏的查询。"""
     return await QueryHistoryService.get_favorites(
-        user_id=current_user.id, page=page, page_size=page_size
+        db=db, user_id=current_user.id, page=page, page_size=page_size
     )
 
 
 @router.get("/statistics")
 async def my_query_statistics(
+    db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
     """获取我的查询统计 — 了解自己的数据使用习惯。"""
-    return await QueryHistoryService.get_query_statistics(user_id=current_user.id)
+    return await QueryHistoryService.get_query_statistics(db=db, user_id=current_user.id)
 
 
 # ── 快捷查询 ──
@@ -70,11 +75,12 @@ async def my_query_statistics(
 async def list_quick_queries(
     db_type: Optional[str] = None,
     keyword: Optional[str] = None,
+    db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
     """获取快捷查询列表（个人+团队共享）。"""
     items = await QueryHistoryService.get_quick_queries(
-        user_id=current_user.id, db_type=db_type, keyword=keyword
+        db=db, user_id=current_user.id, db_type=db_type, keyword=keyword
     )
     return {"items": items, "total": len(items)}
 
@@ -87,10 +93,12 @@ async def create_quick_query(
     description: str = "",
     database_pattern: str = "*",
     is_shared: bool = False,
+    db: AsyncSession = Depends(get_db),
     current_user: User = Depends(get_current_user),
 ):
     """创建快捷查询。"""
     qq = await QueryHistoryService.create_quick_query(
+        db=db,
         name=name,
         description=description,
         query_content=query_content,