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/models/__init__.py

@@ -1,5 +1,13 @@
 """Data models for wikijs-python-sdk."""
 
+from .asset import (
+    Asset,
+    AssetFolder,
+    AssetMove,
+    AssetRename,
+    AssetUpload,
+    FolderCreate,
+)
 from .base import BaseModel
 from .group import (
     Group,
@@ -15,7 +23,13 @@ from .page import Page, PageCreate, PageUpdate
 from .user import User, UserCreate, UserGroup, UserUpdate
 
 __all__ = [
+    "Asset",
+    "AssetFolder",
+    "AssetMove",
+    "AssetRename",
+    "AssetUpload",
     "BaseModel",
+    "FolderCreate",
     "Group",
     "GroupAssignUser",
     "GroupCreate",