feat: push product images to cloud for the QR menu

local_backend now uploads each product's image file to cloud_backend
during the existing ~5 min menu sync, so the QR menu can show real
photos without needing a manually-set digital_image_url. Only
re-uploads images whose content hash changed since the last push
(Product.cloud_image_hash), to avoid re-sending unchanged binaries
every cycle.

Also adds a manual "sync now" trigger (POST /api/system/sync-menu) and
a matching button in Settings → Operation, for pushing menu/price/image
changes immediately instead of waiting for the next automatic cycle.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

local_backend/main.py

@@ -732,6 +732,8 @@ def _run_migrations():
         "ALTER TABLE product_preference_sets ADD COLUMN allow_multi_select INTEGER NOT NULL DEFAULT 0",
         # Per-choice quantity on multi-select preference sets (x2, x3, …)
         "ALTER TABLE product_preference_sets ADD COLUMN allow_choice_quantity INTEGER NOT NULL DEFAULT 0",
+        # Tracks the hash of the product image last pushed to the cloud (Xenia Connect)
+        "ALTER TABLE products ADD COLUMN cloud_image_hash VARCHAR",
     ]
     for sql in migrations:
         try: