fix: Dashboard 页面崩溃修复 + 真实 API 数据对接

- 修复重复 export default 导致页面无法加载
- 添加 dbInstances/tickets/queryStats state 定义
- 对接 /instances /tickets /dashboard/stats 真实 API
- 替换硬编码查询统计数字为 API 驱动
- 修复 calcHealthScore 对缺失字段的安全访问
- 新增 QueryHistory/QuickQuery SQLAlchemy model (建表准备)
- 1040 测试通过

backend/app/models/query_history.py

@@ -0,0 +1,40 @@
+from sqlalchemy import Column, Integer, String, Text, DateTime, Boolean, func
+
+from app.core.database import Base
+
+
+class QueryHistory(Base):
+    __tablename__ = "query_histories"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    user_id = Column(Integer, nullable=False, index=True)
+    username = Column(String(100), nullable=False, default="")
+    instance_id = Column(Integer, nullable=False, default=0)
+    instance_name = Column(String(100), nullable=False, default="")
+    db_type = Column(String(20), nullable=False, default="")
+    database_name = Column(String(100), nullable=False, default="")
+    query_content = Column(Text, nullable=False, default="")
+    query_type = Column(String(50), nullable=False, default="")
+    execution_time_ms = Column(Integer, nullable=False, default=0)
+    result_rows = Column(Integer, nullable=False, default=0)
+    is_success = Column(Boolean, nullable=False, default=True)
+    error_message = Column(Text, nullable=True)
+    is_favorite = Column(Boolean, nullable=False, default=False)
+    note = Column(Text, nullable=False, default="")
+    created_at = Column(DateTime, server_default=func.now())
+
+
+class QuickQueryModel(Base):
+    __tablename__ = "quick_queries"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    name = Column(String(200), nullable=False)
+    description = Column(Text, nullable=False, default="")
+    query_content = Column(Text, nullable=False, default="")
+    db_type = Column(String(20), nullable=False, default="")
+    database_pattern = Column(String(200), nullable=False, default="*")
+    is_shared = Column(Boolean, nullable=False, default=False)
+    created_by = Column(Integer, nullable=False, default=0)
+    created_by_name = Column(String(100), nullable=False, default="")
+    usage_count = Column(Integer, nullable=False, default=0)
+    created_at = Column(DateTime, server_default=func.now())