Implement Assets API with file/asset management operations

Implementation:
- Asset data models (wikijs/models/asset.py)
  - Asset, AssetFolder models
  - AssetUpload, AssetRename, AssetMove models
  - FolderCreate model
  - File size helpers (size_mb, size_kb)
  - Field validation and normalization

- Sync AssetsEndpoint (wikijs/endpoints/assets.py)
  - list(folder_id, kind) - List assets with filtering
  - get(asset_id) - Get single asset
  - rename(asset_id, new_filename) - Rename asset
  - move(asset_id, folder_id) - Move asset between folders
  - delete(asset_id) - Delete asset
  - list_folders() - List all folders
  - create_folder(slug, name) - Create new folder
  - delete_folder(folder_id) - Delete folder
  - Note: upload/download require multipart support (future enhancement)

- Async AsyncAssetsEndpoint (wikijs/aio/endpoints/assets.py)
  - Complete async implementation
  - Identical interface to sync version
  - All asset and folder management operations

- Integration with clients
  - WikiJSClient.assets
  - AsyncWikiJSClient.assets

GraphQL operations for asset and folder management.

🤖 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 GroupsEndpoint, PagesEndpoint, UsersEndpoint
+from .endpoints import AssetsEndpoint, GroupsEndpoint, PagesEndpoint, UsersEndpoint
 from .exceptions import (
     APIError,
     AuthenticationError,
@@ -92,8 +92,7 @@ class WikiJSClient:
         self.pages = PagesEndpoint(self)
         self.users = UsersEndpoint(self)
         self.groups = GroupsEndpoint(self)
-        # Future endpoints:
-        # self.assets = AssetsEndpoint(self)
+        self.assets = AssetsEndpoint(self)
 
     def _create_session(self) -> requests.Session:
         """Create configured HTTP session with retry strategy.