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/client.py

@@ -8,7 +8,7 @@ from requests.adapters import HTTPAdapter
 from urllib3.util.retry import Retry
 
 from .auth import APIKeyAuth, AuthHandler
-from .endpoints import PagesEndpoint, UsersEndpoint
+from .endpoints import GroupsEndpoint, PagesEndpoint, UsersEndpoint
 from .exceptions import (
     APIError,
     AuthenticationError,
@@ -91,8 +91,9 @@ class WikiJSClient:
         # Endpoint handlers
         self.pages = PagesEndpoint(self)
         self.users = UsersEndpoint(self)
+        self.groups = GroupsEndpoint(self)
         # Future endpoints:
-        # self.groups = GroupsEndpoint(self)
+        # self.assets = AssetsEndpoint(self)
 
     def _create_session(self) -> requests.Session:
         """Create configured HTTP session with retry strategy.