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/models/backup.py

@@ -0,0 +1,19 @@
+from sqlalchemy import Column, Integer, String, BigInteger, Text, DateTime, func
+
+from app.core.database import Base
+
+
+class Backup(Base):
+    __tablename__ = "backups"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    instance_id = Column(Integer, nullable=False)
+    title = Column(String(200), nullable=False)
+    database = Column(String(100), nullable=False)
+    filename = Column(String(500), nullable=False, default="")
+    size_bytes = Column(BigInteger, default=0)
+    status = Column(String(20), default="pending")  # pending / running / completed / failed
+    created_by = Column(Integer, nullable=True)
+    created_at = Column(DateTime, server_default=func.now())
+    completed_at = Column(DateTime, nullable=True)
+    remark = Column(Text, nullable=True)