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/models/__init__.py

@@ -1,11 +1,29 @@
 """Data models for wikijs-python-sdk."""
 
 from .base import BaseModel
+from .group import (
+    Group,
+    GroupAssignUser,
+    GroupCreate,
+    GroupPageRule,
+    GroupPermission,
+    GroupUnassignUser,
+    GroupUpdate,
+    GroupUser,
+)
 from .page import Page, PageCreate, PageUpdate
 from .user import User, UserCreate, UserGroup, UserUpdate
 
 __all__ = [
     "BaseModel",
+    "Group",
+    "GroupAssignUser",
+    "GroupCreate",
+    "GroupPageRule",
+    "GroupPermission",
+    "GroupUnassignUser",
+    "GroupUpdate",
+    "GroupUser",
     "Page",
     "PageCreate",
     "PageUpdate",