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/main.py

@@ -9,6 +9,7 @@ from starlette.middleware.sessions import SessionMiddleware
 from web.auth import get_current_user
 from web.config import settings
 from web.health_checker import health_check_loop
+from web.sync_engine import sync_loop
 from web.models import User
 from web.routes import auth, profile, reset, evotor, vk, sync, catalog
 from web.routes import connections
@@ -16,13 +17,18 @@ from web.routes import connections
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    task = asyncio.create_task(health_check_loop(settings.HEALTH_CHECK_INTERVAL_SECONDS))
+    tasks = [
+        asyncio.create_task(health_check_loop(settings.HEALTH_CHECK_INTERVAL_SECONDS)),
+        asyncio.create_task(sync_loop(settings.SYNC_INTERVAL_SECONDS)),
+    ]
     yield
-    task.cancel()
-    try:
-        await task
-    except asyncio.CancelledError:
-        pass
+    for t in tasks:
+        t.cancel()
+    for t in tasks:
+        try:
+            await t
+        except asyncio.CancelledError:
+            pass
 
 
 app = FastAPI(title="ЭВОСИНК — Личный кабинет", lifespan=lifespan)