feat: добавить версионирование source image

- добавлен endpoint создания новой версии asset
- реализовано переключение currentVersion через image_asset_versions
- обновлена документация business API и dev smoke flow

docs/backend-contract-draft.md

@@ -110,12 +110,13 @@ DELETE /allowed-hosts/:id
 POST /assets
 GET /assets
 GET /assets/:publicId
+POST /assets/:publicId/versions
 GET /assets/:publicId/variants
 POST /assets/:publicId/variants
 DELETE /assets/:id
 ```
 
-Сейчас реализованы `POST /assets`, `GET /assets`, `GET /assets/:publicId`, `GET /assets/:publicId/variants`, `POST /assets/:publicId/variants`.
+Сейчас реализованы `POST /assets`, `GET /assets`, `GET /assets/:publicId`, `POST /assets/:publicId/versions`, `GET /assets/:publicId/variants`, `POST /assets/:publicId/variants`.
 
 `POST /assets` request:
 
@@ -167,6 +168,32 @@ Responsibilities:
 
 `GET /assets/:publicId/variants` возвращает rows из `image_variants` с public Gateway URL, S3 key и status.
 
+`POST /assets/:publicId/versions` request:
+
+```json
+{
+  "sourceUrl": "https://storage.yandexcloud.net/shared1318/img/1.jpg"
+}
+```
+
+Response:
+
+```json
+{
+  "assetId": "59fcf4f6-9891-4df4-8bb7-a4dbe570bb66",
+  "versionId": "3b5da974-bb7f-4d73-b172-d6ad9c244528",
+  "publicId": "asset_demo",
+  "previousVersion": 1,
+  "version": 2,
+  "sourceUrl": "https://storage.yandexcloud.net/shared1318/img/1.jpg",
+  "sourceHost": "storage.yandexcloud.net",
+  "imageBasePath": "/images/asset_demo/v2/card",
+  "createdAt": "2026-05-05T12:00:00.000Z"
+}
+```
+
+Новая версия становится `currentVersion` asset. Старые Gateway URLs с `/v1/` остаются immutable и не требуют purge.
+
 ## Variants
 
 ```text