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/migrations/versions/a7b8c9d0e1f2_add_synced_at_to_cached_products.py

@@ -0,0 +1,26 @@
+"""add synced_at to cached_products
+
+Revision ID: a7b8c9d0e1f2
+Revises: f6a7b8c9d0e1
+Branch Labels: None
+Depends On: None
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "a7b8c9d0e1f2"
+down_revision = "f6a7b8c9d0e1"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "cached_products",
+        sa.Column("synced_at", sa.DateTime(), nullable=True),
+    )
+
+
+def downgrade():
+    op.drop_column("cached_products", "synced_at")