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/sql_assistant.py

@@ -1,11 +1,20 @@
 """SQL助手 API — 人性化辅助功能。"""
+import time
 from typing import Optional
+
 from fastapi import APIRouter, Depends, HTTPException
 from pydantic import BaseModel
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.core.database import get_db
+from app.core.crypto import decrypt_password
+from app.core.permissions import check_instance_access, check_instance_permission
 from app.core.security import get_current_user
 from app.models.user import User
+from app.models.instance import Instance
 from app.services.sql_assistant import SQLAssistant
+from app.services.explain_service import ExplainService
 
 router = APIRouter(prefix="/sql-assistant", tags=["SQL助手"])
 
@@ -111,3 +120,63 @@ async def parse_explain(
         "warnings": result.warnings,
         "human_readable": result.human_readable,
     }
+
+
+# ===== EXPLAIN 真实执行 API =====
+
+
+class ExplainRequest(BaseModel):
+    """EXPLAIN 请求体。"""
+    instance_id: int
+    database: str
+    query: str
+
+
+@router.post("/explain")
+async def execute_explain(
+    req: ExplainRequest,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """连接数据源执行 EXPLAIN 并解析为统一树结构。
+
+    根据数据源类型自动构造对应的 EXPLAIN 语句:
+    - MySQL: EXPLAIN FORMAT=JSON <query>
+    - PostgreSQL: EXPLAIN (FORMAT JSON, ANALYZE false) <query>
+    - SQLite: EXPLAIN QUERY PLAN <query>
+
+    返回统一的树形执行计划、优化警告和汇总统计。
+    """
+    if current_user.role == "guest":
+        raise HTTPException(status_code=403, detail="访客无权执行 EXPLAIN，请联系管理员申请权限")
+
+    if not req.query or not req.query.strip():
+        raise HTTPException(status_code=400, detail="查询语句不能为空")
+
+    # 获取实例
+    result = await db.execute(select(Instance).where(Instance.id == req.instance_id))
+    instance = result.scalar_one_or_none()
+    if not instance:
+        raise HTTPException(status_code=404, detail="数据源不存在，请确认实例ID正确")
+
+    # 权限检查
+    await check_instance_access(current_user, instance, db)
+    await check_instance_permission(current_user, instance, "query:select", db)
+
+    # 解密密码
+    instance.password = decrypt_password(instance.password)
+
+    start_time = time.time()
+    try:
+        explain_result = await ExplainService.execute_explain(
+            instance, req.database, req.query
+        )
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
+    except Exception as e:
+        raise HTTPException(status_code=500, detail=f"EXPLAIN 执行失败: {e}")
+
+    elapsed_ms = int((time.time() - start_time) * 1000)
+    explain_result["elapsed_ms"] = elapsed_ms
+
+    return explain_result