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/templates/catalog_products.html

@@ -40,6 +40,7 @@
                 <th>Кол-во</th>
                 <th>Ед. изм.</th>
                 <th>В продаже</th>
+                <th>Синхронизирован</th>
                 <th>Фильтр</th>
                 <th></th>
             </tr>
@@ -62,6 +63,15 @@
                         <i class="bi bi-x-circle-fill text-danger"></i>
                     {% endif %}
                 </td>
+                <td>
+                    {% if product.synced_at %}
+                        <span title="{{ product.synced_at.strftime('%d.%m.%Y %H:%M') }}">
+                            <i class="bi bi-check-circle-fill text-success"></i>
+                        </span>
+                    {% else %}
+                        <span class="text-muted">—</span>
+                    {% endif %}
+                </td>
                 <td>
                     {% if mode == "include" %}
                         <span class="badge bg-success">✓ Включено</span>