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/models/product.py

@@ -45,6 +45,9 @@ class Product(Base):
     digital_price = Column(Float, nullable=True)
     digital_discount = Column(Float, default=0.0, nullable=False)
     digital_image_url = Column(String, nullable=True)
+    # sha256 of the image_url file last successfully pushed to the cloud — lets
+    # cloud_sync skip re-uploading unchanged images on every sync tick
+    cloud_image_hash = Column(String, nullable=True)
 
     # Unit of measure: "piece" | "portion" | "kg" | "liter" | "gram" | "ml"
     unit_type = Column(String, default="piece", nullable=False)