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/endpoints/__init__.py

@@ -7,18 +7,20 @@ Implemented:
 - Pages API (CRUD operations) ✅
 - Users API (user management) ✅
 - Groups API (group management) ✅
+- Assets API (file/asset management) ✅
 
 Future implementations:
-- Assets API (file management)
 - System API (system information)
 """
 
+from .assets import AssetsEndpoint
 from .base import BaseEndpoint
 from .groups import GroupsEndpoint
 from .pages import PagesEndpoint
 from .users import UsersEndpoint
 
 __all__ = [
+    "AssetsEndpoint",
     "BaseEndpoint",
     "GroupsEndpoint",
     "PagesEndpoint",