refactor: 数据源菜单合一+布局优化

- 4个数据源菜单(MySQL/PG/Redis/Mongo)合并为1个「数据源」
- InstancePage 改为统一入口，顶部Tab切换类型过滤
- 添加数据源改为弹框Modal（默认MySQL）
- 去掉左下角用户区，退出登录移到右上角header
- 侧边栏收窄220px，整体布局更紧凑
- 后端202 tests passing
- 前端构建零错误

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

@@ -0,0 +1,70 @@
+"""数据字典 API 端点。"""
+from typing import Optional
+
+from fastapi import APIRouter, Depends, HTTPException, Query
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.database import get_db
+from app.core.security import require_role
+from app.models.user import User
+from app.services import dictionary_service
+
+router = APIRouter(tags=["数据字典"])
+
+
+@router.get("/instances/{instance_id}/databases")
+async def list_databases(
+    instance_id: int,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(require_role("admin", "dba", "developer")),
+):
+    """获取实例的数据库列表。"""
+    databases = await dictionary_service.list_databases(db, instance_id)
+    if databases is None:
+        raise HTTPException(status_code=404, detail="数据源不存在")
+    return {"databases": databases}
+
+
+@router.get("/instances/{instance_id}/tables")
+async def list_tables(
+    instance_id: int,
+    database: str = Query(..., description="数据库名"),
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(require_role("admin", "dba", "developer")),
+):
+    """获取指定数据库的表列表。"""
+    tables = await dictionary_service.list_tables(db, instance_id, database)
+    if tables is None:
+        raise HTTPException(status_code=404, detail="数据源不存在")
+    return {"tables": tables}
+
+
+@router.get("/instances/{instance_id}/tables/{table_name}")
+async def get_table_detail(
+    instance_id: int,
+    table_name: str,
+    database: str = Query(..., description="数据库名"),
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(require_role("admin", "dba", "developer")),
+):
+    """获取表详情: 字段/索引/DDL/行数。"""
+    detail = await dictionary_service.get_table_detail(
+        db, instance_id, database, table_name
+    )
+    if not detail:
+        raise HTTPException(status_code=404, detail="数据源不存在")
+    return detail
+
+
+@router.get("/instances/{instance_id}/export")
+async def export_dictionary(
+    instance_id: int,
+    database: str = Query(..., description="数据库名"),
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(require_role("admin", "dba", "developer")),
+):
+    """导出整个数据库的表结构信息。"""
+    result = await dictionary_service.export_dictionary(
+        db, instance_id, database
+    )
+    return {"tables": result}