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/models.py

@@ -149,6 +149,7 @@ class CachedProduct(Base):
     article_number = Column(String(100), nullable=True)
     allow_to_sell = Column(Boolean, nullable=True)
     fetched_at = Column(DateTime, nullable=False)
+    synced_at = Column(DateTime, nullable=True)
 
     __table_args__ = (
         UniqueConstraint("user_id", "evotor_id"),