Add sync engine and wire it into the web app

- Add sync_engine.py: background asyncio loop syncing Evotor products to VK market
- Wire sync_loop into lifespan alongside health_check_loop
- Add SYNC_INTERVAL_SECONDS and VK_DEFAULT_PHOTO_PATH settings to config
- Mount default product image in docker-compose
- Add synced_at column to CachedProduct model + migration
- Show synced_at status in catalog products template
- Fix VK groups API response parsing (handle list vs dict)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -19,6 +19,7 @@ services:
       - ./web:/app/web
       - ./alembic.ini:/app/alembic.ini
       - ./docker-entrypoint.sh:/app/docker-entrypoint.sh
+      - ./5393364294319597854.png:/app/default_product.png:ro
     restart: unless-stopped
     extra_hosts:
       - "host.docker.internal:host-gateway"