fix: QR menu branding fields — display name, header image, empty-to-hide

- menu_display_name is now separate from the site's internal name (was
  incorrectly showing the sysadmin-only site name on the public menu).
- Header image now resolves against the cloud API origin instead of
  the menu-app's own origin, fixing the broken-image icon.
- Tagline/blurb/hours now genuinely hide when cleared: the sysadmin
  save handler was converting empty fields to null, which the PUT
  endpoint's exclude_none silently drops instead of clearing.
- Added a settable blurb (EN/GR) below the tagline, previously
  hardcoded ("Fresh seasonal plates, served with care.").
- Collapsed hours from two fields (EN/GR) to one — opening hours don't
  vary by language.

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

cloud_backend/routers/menu.py

@@ -39,9 +39,10 @@ def get_menu(site_slug: str, db: Session = Depends(get_db)):
     data = json.loads(snapshot.snapshot_json)
     data["menu_mode"] = site.menu_mode
     data["restaurant"] = {
-        "name": site.name,
+        "name": site.menu_display_name,
         "tagline": {"en": site.menu_tagline_en, "gr": site.menu_tagline_gr},
-        "hours": {"en": site.menu_hours_en, "gr": site.menu_hours_gr},
+        "blurb": {"en": site.menu_blurb_en, "gr": site.menu_blurb_gr},
+        "hours": site.menu_hours,
         "headerImageUrl": site.menu_header_image_url,
     }
     return data