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/endpoints/__init__.py

@@ -1,11 +1,13 @@
 """Async endpoint handlers for Wiki.js API."""
 
 from .base import AsyncBaseEndpoint
+from .groups import AsyncGroupsEndpoint
 from .pages import AsyncPagesEndpoint
 from .users import AsyncUsersEndpoint
 
 __all__ = [
     "AsyncBaseEndpoint",
+    "AsyncGroupsEndpoint",
     "AsyncPagesEndpoint",
     "AsyncUsersEndpoint",
 ]