feat: добавить генерацию image variants

- добавлен shared config presets, custom transforms и allowlist hosts
- реализованы Backend endpoints для assets, presets и variants
- добавлена orchestration через PostgreSQL, RabbitMQ, S3 и worker
- обновлён Gateway read-through flow с L1 cache и корректным Vary: Accept
- добавлена миграция resize_mode для variants lookup
- обновлены dev scripts, env template, lockfile и документация

.env.example

@@ -29,6 +29,14 @@ PUBLIC_IMAGE_BASE_URL=http://localhost:8888
 
 # Gateway proxies /api and Swagger routes to this upstream.
 GATEWAY_BACKEND_UPSTREAM=http://localhost:3001
+GATEWAY_L1_MAX_ENTRIES=256
+GATEWAY_L1_TTL_MS=600000
+
+# MVP dev mode: mock source host allowlist without DB/admin CRUD.
+SOURCE_HOST_ALLOW_ALL=false
+SOURCE_ALLOWED_HOSTS=storage.yandexcloud.net
+IMAGE_ALLOW_CUSTOM_TRANSFORMS=true
+IMAGE_ENSURE_WAIT_MS=15000
 
 # Dev imgproxy is exposed only on localhost.
 IMGPROXY_PORT=18080