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/main.py

@@ -13,6 +13,7 @@ import models.menu_snapshot    # noqa: F401
 import models.online_order     # noqa: F401
 import models.manager_account  # noqa: F401
 import models.stats_snapshot   # noqa: F401
+import models.product_image    # noqa: F401
 
 from routers import auth, sites, heartbeat
 from routers import menu as menu_router
@@ -90,6 +91,9 @@ app.include_router(remote_dashboard_router.router,prefix="/api/remote",    tags=
 os.makedirs("/app/data/site_headers", exist_ok=True)
 app.mount("/static/site_headers", StaticFiles(directory="/app/data/site_headers"), name="site_headers")
 
+os.makedirs("/app/data/product_images", exist_ok=True)
+app.mount("/static/product_images", StaticFiles(directory="/app/data/product_images"), name="product_images")
+
 
 @app.get("/health")
 def health():