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/aio/client.py

@@ -27,7 +27,7 @@ from ..utils import (
     parse_wiki_response,
 )
 from ..version import __version__
-from .endpoints import AsyncGroupsEndpoint, AsyncPagesEndpoint, AsyncUsersEndpoint
+from .endpoints import AsyncAssetsEndpoint, AsyncGroupsEndpoint, AsyncPagesEndpoint, AsyncUsersEndpoint
 
 
 class AsyncWikiJSClient:
@@ -105,8 +105,7 @@ class AsyncWikiJSClient:
         self.pages = AsyncPagesEndpoint(self)
         self.users = AsyncUsersEndpoint(self)
         self.groups = AsyncGroupsEndpoint(self)
-        # Future endpoints:
-        # self.assets = AsyncAssetsEndpoint(self)
+        self.assets = AsyncAssetsEndpoint(self)
 
     def _get_session(self) -> aiohttp.ClientSession:
         """Get or create aiohttp session.