refactor: 权限组从一组一类型改为一组同时包含readonly+write实例

- 新增 instance_group_members 中间表关联权限组和实例
- InstanceGroup 模型移除旧的 instances relationship，改为通过中间表查询
- Instance 模型保留 group_id FK（兼容）但移除 group relationship
- Schema 新增 readonly_instance_ids / write_instance_ids 字段
- InstanceGroupResponse 改为 readonly_instance_count + write_instance_count
- InstanceGroupDetailResponse 返回 readonly_instances + write_instances
- InstanceAssignRequest 新增 access_type 参数
- API create/update 支持同步写入 instance_group_members
- role_permissions 服务更新为使用中间表查询
- 新增迁移 SQL 并应用到 sqlite3.db
- 测试全部通过 (20/20)

backend/tests/conftest.py

@@ -9,7 +9,7 @@ from app.core.database import Base, get_db
 from app.core.security import hash_password, create_access_token
 from app.models.user import User
 from app.models.instance import Instance
-from app.models.instance_group import InstanceGroup, user_groups
+from app.models.instance_group import InstanceGroup, user_groups, instance_group_members
 from app.models.ticket import Ticket
 from app.models.audit_log import AuditLog
 from app.models.audit_rule import AuditRule