New endpoints — Assets with a required asset type

Added GET /v2/assets and POST /v2/assets. Both require an asset_type of owned or associated. Use GET /v2/assets to list assets of one type, and POST /v2/assets to create an asset of that type. The list supports the existing platform_name, brand_ids, brand_names, limit, and offset filters. Every asset in the response includes its asset_type.

Create returns 201 with the new asset. A request that omits asset_type returns 422. A request that names a brand your organization cannot read returns 404.

These endpoints are available on V2 only.