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>

web/config.py

@@ -14,6 +14,9 @@ class Settings(BaseSettings):
 
     HEALTH_CHECK_INTERVAL_SECONDS: int = 600
     CATALOG_REFRESH_INTERVAL_SECONDS: int = 3600
+    SYNC_INTERVAL_SECONDS: int = 3600
+
+    VK_DEFAULT_PHOTO_PATH: str = "/app/default_product.png"
 
     VK_API_VERSION: str = "5.131"