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

docs/backend-contract-draft.md

@@ -111,12 +111,13 @@ POST /assets
 GET /assets
 GET /assets/:publicId
 POST /assets/:publicId/versions
+GET /assets/:publicId/picture
 GET /assets/:publicId/variants
 POST /assets/:publicId/variants
 DELETE /assets/:id
 ```
 
-Сейчас реализованы `POST /assets`, `GET /assets`, `GET /assets/:publicId`, `POST /assets/:publicId/versions`, `GET /assets/:publicId/variants`, `POST /assets/:publicId/variants`.
+Сейчас реализованы `POST /assets`, `GET /assets`, `GET /assets/:publicId`, `POST /assets/:publicId/versions`, `GET /assets/:publicId/picture`, `GET /assets/:publicId/variants`, `POST /assets/:publicId/variants`.
 
 `POST /assets` request:
 
@@ -194,6 +195,35 @@ Response:
 
 Новая версия становится `currentVersion` asset. Старые Gateway URLs с `/v1/` остаются immutable и не требуют purge.
 
+`GET /assets/:publicId/picture?preset=card&sizes=100vw` response:
+
+```json
+{
+  "publicId": "asset_demo",
+  "preset": "card",
+  "version": 2,
+  "quality": 80,
+  "sizes": "100vw",
+  "widths": [320, 640, 960],
+  "image": {
+    "src": "http://localhost:8888/images/asset_demo/v2/card?w=960&q=80&f=jpg",
+    "format": "jpg",
+    "type": "image/jpeg",
+    "width": 960,
+    "height": 0
+  },
+  "sources": [
+    {
+      "format": "avif",
+      "type": "image/avif",
+      "srcSet": "http://localhost:8888/images/asset_demo/v2/card?w=320&q=80&f=avif 320w, http://localhost:8888/images/asset_demo/v2/card?w=640&q=80&f=avif 640w, http://localhost:8888/images/asset_demo/v2/card?w=960&q=80&f=avif 960w"
+    }
+  ]
+}
+```
+
+Endpoint не ставит generation jobs: Gateway lazy path сгенерирует bytes на первом запросе или отдаст cache/S3.
+
 ## Variants
 
 ```text