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>

connect_frontend/menu-app/src/pages/MenuPage.jsx

@@ -54,6 +54,13 @@ const I18N = {
   },
 }
 
+// Resolve a possibly-relative path (e.g. "/static/product_images/x.jpg") against
+// the cloud API origin. Absolute URLs (manager-set external overrides) pass through untouched.
+function resolveImageUrl(url) {
+  if (!url) return null
+  return /^https?:\/\//i.test(url) ? url : `${CLOUD_URL}${url}`
+}
+
 // ── Normalise a backend product to the shape the UI expects ──────────────────
 function normaliseProduct(p, catId) {
   return {
@@ -68,7 +75,7 @@ function normaliseProduct(p, catId) {
     allergens: p.allergens || [],
     ingredients: p.ingredients || null,
     discountPct: p.digital_discount || p.discountPct || 0,
-    image_url: p.digital_image_url || p.image_url || null,
+    image_url: resolveImageUrl(p.digital_image_url || p.image_url || null),
     digital_available: p.digital_available !== false,
   }
 }