feat: добавить endpoint picture/srcset

- добавлен contract DTO для picture sources и fallback image
- реализована выдача versioned Gateway URLs по static presets
- обновлена документация business API и dev smoke flow

README.md

@@ -73,6 +73,7 @@ Business API без админки:
 ```bash
 curl -sS http://localhost:3001/api/presets
 curl -sS http://localhost:3001/api/assets
+curl -sS 'http://localhost:3001/api/assets/asset_demo/picture?preset=card&sizes=100vw'
 curl -sS http://localhost:3001/api/assets/asset_demo/variants
 curl -sS -X POST http://localhost:3001/api/assets/asset_demo/versions \
   -H 'content-type: application/json' \