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/models/site.py

@@ -26,8 +26,10 @@ class Site(Base):
 
     # QR menu branding/config
     menu_mode = Column(String, nullable=False, default="order")  # "order" | "view_only"
+    menu_display_name = Column(String, nullable=True)
     menu_tagline_en = Column(String, nullable=True)
     menu_tagline_gr = Column(String, nullable=True)
-    menu_hours_en = Column(String, nullable=True)
-    menu_hours_gr = Column(String, nullable=True)
+    menu_blurb_en = Column(String, nullable=True)
+    menu_blurb_gr = Column(String, nullable=True)
+    menu_hours = Column(String, nullable=True)
     menu_header_image_url = Column(String, nullable=True)