修复：解决后端循环导入问题

问题：Docker 启动时后端服务崩溃，提示循环导入错误
原因：permission_cache.py 在模块级别导入 project_permissions.py，而后者也导入前者
解决：使用延迟导入，在方法内部导入所需的函数

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/app/core/permission_cache.py

@@ -12,11 +12,6 @@ from typing import Any
 
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.core.project_permissions import (
-    get_project_role_permissions,
-    get_member_role,
-)
-
 
 class PermissionCache:
     """权限缓存管理器
@@ -63,6 +58,8 @@ class PermissionCache:
         Returns:
             权限矩阵：{role: {module: {action: bool}}}
         """
+        from app.core.project_permissions import get_project_role_permissions as _get_project_role_permissions
+
         if ttl is None:
             ttl = self.default_ttl
 
@@ -75,7 +72,7 @@ class PermissionCache:
                 return cached_data
 
         # 缓存未命中，从数据库查询
-        permissions = await get_project_role_permissions(db, study_id)
+        permissions = await _get_project_role_permissions(db, study_id)
 
         # 存储到缓存
         self._project_permissions_cache[cache_key] = (permissions, time.time())
@@ -100,6 +97,8 @@ class PermissionCache:
         Returns:
             成员在项目中的角色，如果不是成员则返回 None
         """
+        from app.core.project_permissions import get_member_role as _get_member_role
+
         if ttl is None:
             ttl = self.default_ttl
 
@@ -112,7 +111,7 @@ class PermissionCache:
                 return cached_role
 
         # 缓存未命中，从数据库查询
-        role = await get_member_role(db, study_id, user_id)
+        role = await _get_member_role(db, study_id, user_id)
 
         # 存储到缓存
         self._member_role_cache[cache_key] = (role, time.time())