Implement Groups API with complete CRUD operations

Implementation:
- Group data models (wikijs/models/group.py)
  - Group, GroupCreate, GroupUpdate models
  - GroupPermission, GroupPageRule, GroupUser models
  - GroupAssignUser, GroupUnassignUser models
  - Field validation and normalization

- Sync GroupsEndpoint (wikijs/endpoints/groups.py)
  - list() - List all groups with users
  - get(group_id) - Get single group
  - create(group_data) - Create new group
  - update(group_id, group_data) - Update existing group
  - delete(group_id) - Delete group
  - assign_user(group_id, user_id) - Add user to group
  - unassign_user(group_id, user_id) - Remove user from group

- Async AsyncGroupsEndpoint (wikijs/aio/endpoints/groups.py)
  - Complete async implementation
  - Identical interface to sync version
  - All CRUD operations + user management

- Integration with clients
  - WikiJSClient.groups
  - AsyncWikiJSClient.groups

GraphQL operations for all group management features.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

wikijs/aio/client.py

@@ -27,7 +27,7 @@ from ..utils import (
     parse_wiki_response,
 )
 from ..version import __version__
-from .endpoints import AsyncPagesEndpoint, AsyncUsersEndpoint
+from .endpoints import AsyncGroupsEndpoint, AsyncPagesEndpoint, AsyncUsersEndpoint
 
 
 class AsyncWikiJSClient:
@@ -104,8 +104,9 @@ class AsyncWikiJSClient:
         # Endpoint handlers (will be initialized when session is created)
         self.pages = AsyncPagesEndpoint(self)
         self.users = AsyncUsersEndpoint(self)
+        self.groups = AsyncGroupsEndpoint(self)
         # Future endpoints:
-        # self.groups = AsyncGroupsEndpoint(self)
+        # self.assets = AsyncAssetsEndpoint(self)
 
     def _get_session(self) -> aiohttp.ClientSession:
         """Get or create aiohttp session.