feat: 实例权限组系统 — 完整CRUD + 实例/用户关联 + 前端管理页面

后端:
- InstanceGroup 模型 (name/db_type/permissions JSON/is_active)
- user_groups 关联表 (用户-权限组 M2M + 权限覆盖)
- Instance 模型增加 group_id FK
- User 模型增加 groups 关系
- API: CRUD + 实例关联 + 用户分配 + 权限组过滤
- 15 个新测试 (CRUD/关联/权限检查/参数校验) — 217/217 passing

前端:
- InstanceGroupPage: 左右布局(列表+详情)
- 权限配置: 5类17项权限勾选 (查询/DDL/工单/数据操作/管理)
- 实例关联/用户关联弹框
- 侧边栏增加权限组菜单 (数据源分组下)
- 路由 + 权限注册

backend/app/schemas/instance_group.py

@@ -0,0 +1,53 @@
+from datetime import datetime
+from typing import Optional, List
+from pydantic import BaseModel, Field
+
+
+class InstanceGroupCreate(BaseModel):
+    name: str = Field(..., min_length=1, max_length=100)
+    description: Optional[str] = None
+    db_type: str = Field(default="all", pattern=r"^(mysql|redis|pg|mongo|all)$")
+    permissions: Optional[List[str]] = None
+    is_active: bool = True
+
+
+class InstanceGroupUpdate(BaseModel):
+    name: Optional[str] = Field(default=None, min_length=1, max_length=100)
+    description: Optional[str] = None
+    db_type: Optional[str] = Field(default=None, pattern=r"^(mysql|redis|pg|mongo|all)$")
+    permissions: Optional[List[str]] = None
+    is_active: Optional[bool] = None
+
+
+class InstanceGroupResponse(BaseModel):
+    id: int
+    name: str
+    description: Optional[str] = None
+    db_type: str
+    permissions: Optional[List[str]] = None
+    is_active: bool
+    created_at: Optional[datetime] = None
+    updated_at: Optional[datetime] = None
+    instance_count: int = 0
+    user_count: int = 0
+
+    class Config:
+        from_attributes = True
+
+
+class InstanceGroupDetailResponse(InstanceGroupResponse):
+    instances: List[dict] = []
+    users: List[dict] = []
+
+
+class InstanceAssignRequest(BaseModel):
+    instance_ids: List[int] = Field(..., min_length=1)
+
+
+class UserAssignRequest(BaseModel):
+    user_ids: List[int] = Field(..., min_length=1)
+    permissions: Optional[List[str]] = None
+
+
+class UserGroupUpdateRequest(BaseModel):
+    group_ids: List[int] = Field(default_factory=list)