feat: receive and serve product images pushed from local sites

New POST /api/menu/sync-image (site-authenticated) accepts a product's
image file, stores it under /app/data/product_images keyed by
(site, product_id), and skips the write entirely if the uploaded
content hash matches what's already stored. GET /api/menu/{site_slug}
now injects the cloud-hosted image URL into each product that has no
manual digital_image_url override — replacing the local-only image_url
from the snapshot, which was never reachable from the public internet.

Also fixes the menu-app resolving product image URLs as bare relative
paths instead of prefixing them with the cloud API origin (same class
of bug as the earlier header-image fix).

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

cloud_backend/models/product_image.py

@@ -0,0 +1,19 @@
+from sqlalchemy import Column, Integer, String, DateTime, ForeignKey, UniqueConstraint
+from sqlalchemy.sql import func
+from database import Base
+
+
+class ProductImage(Base):
+    """Cloud-hosted copy of a local product's image, pushed by local_backend
+    during menu sync. Used as a fallback on the public QR menu when the
+    product has no digital_image_url override set."""
+    __tablename__ = "product_images"
+
+    id = Column(Integer, primary_key=True, index=True)
+    site_id = Column(Integer, ForeignKey("sites.id"), nullable=False, index=True)
+    product_id = Column(Integer, nullable=False)
+    image_url = Column(String, nullable=False)
+    image_hash = Column(String, nullable=False)
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    __table_args__ = (UniqueConstraint("site_id", "product_id", name="uq_product_images_site_product"),)