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/schemas/site.py

@@ -16,10 +16,12 @@ class SiteUpdate(BaseModel):
     license_expires_at: datetime | None = None
     waiter_domain: str | None = None
     menu_mode: str | None = None
+    menu_display_name: str | None = None
     menu_tagline_en: str | None = None
     menu_tagline_gr: str | None = None
-    menu_hours_en: str | None = None
-    menu_hours_gr: str | None = None
+    menu_blurb_en: str | None = None
+    menu_blurb_gr: str | None = None
+    menu_hours: str | None = None
 
 
 class SiteOut(BaseModel):
@@ -38,10 +40,12 @@ class SiteOut(BaseModel):
     last_seen_local_ip: str | None
     waiter_domain: str | None
     menu_mode: str
+    menu_display_name: str | None
     menu_tagline_en: str | None
     menu_tagline_gr: str | None
-    menu_hours_en: str | None
-    menu_hours_gr: str | None
+    menu_blurb_en: str | None
+    menu_blurb_gr: str | None
+    menu_hours: str | None
     menu_header_image_url: str | None
 
     model_config = {"from_attributes": True}