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/schemas/backup.py

@@ -0,0 +1,28 @@
+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class BackupCreate(BaseModel):
+    title: str
+    instance_id: int
+    database: str
+    remark: Optional[str] = None
+
+
+class BackupResponse(BaseModel):
+    id: int
+    title: str
+    instance_id: int
+    database: str
+    filename: str
+    size_bytes: int
+    status: str
+    created_by: Optional[int] = None
+    created_at: Optional[datetime] = None
+    completed_at: Optional[datetime] = None
+    remark: Optional[str] = None
+
+    class Config:
+        from_attributes = True