fix(branding): make 'Show logo in hero' a true global toggle with per-event override (#756)

Before: the global branding_logo_display_hero toggle was only a
creation-time default — snapshotted into each event's hero_logo_visible
column at creation and never consulted again. Disabling it did nothing
to existing galleries (the reporter's bug), and the two gallery render
paths disagreed (GalleryLayout read the global, HeroHeader read the
per-event snapshot).

Now: NULL per-event hero_logo_visible = 'inherit the global toggle';
an explicit true/false is a per-gallery override.

- Migration 152: make events.hero_logo_visible nullable and NULL out the
  defaulted  rows so existing galleries follow the global going
  forward. Deliberate per-gallery hides () are preserved.
- Creation stores NULL unless the admin explicitly sets it; the update
  path preserves NULL.
- gallery.js resolves per-event ?? global (branding_logo_display_hero,
  default true) and sends the EFFECTIVE value on both gallery responses.
- Both frontend render paths now consume that resolved value
  (GalleryLayout gets it via a new heroLogoVisible prop).
- Admin per-event control is now tri-state: Use branding default /
  Always show / Always hide (en + de).

Verified: SQLite migration + live resolution (inherit follows global
both ways; override wins both ways); PG migration SQL dry-run; the admin
tri-state renders 'Use branding default' for an inherited event; tsc
clean, 106 adminEvents+gallery tests pass, build green.

frontend/src/components/gallery/GalleryLayout.tsx

@@ -25,6 +25,10 @@ interface GalleryLayoutProps {
     promo_mode?: 'inherit' | 'custom' | 'off';
     promo_markdown?: string | null;
   };
+  // Effective hero-logo visibility for THIS gallery, already resolved by the
+  // backend (per-event override, else the global branding toggle) (#756).
+  // When provided it wins over brandingSettings.logo_display_hero.
+  heroLogoVisible?: boolean;
   brandingSettings?: {
     company_name?: string;
     company_tagline?: string;
@@ -108,6 +112,7 @@ const HeaderDownloadButton: React.FC<{
 export const GalleryLayout: React.FC<GalleryLayoutProps> = ({
   event,
   brandingSettings,
+  heroLogoVisible,
   showLogout = false,
   onLogout,
   showDownloadAll = false,
@@ -202,6 +207,9 @@ export const GalleryLayout: React.FC<GalleryLayoutProps> = ({
     if (context === 'header') {
       return brandingSettings?.logo_display_header !== false;
     } else {
+      // #756: prefer the backend-resolved per-event value (override, else
+      // global); fall back to the global branding toggle if not provided.
+      if (heroLogoVisible !== undefined) return heroLogoVisible;
       return brandingSettings?.logo_display_hero !== false;
     }
   };

frontend/src/components/gallery/GalleryView.tsx

@@ -820,6 +820,7 @@ export const GalleryView: React.FC<GalleryViewProps> = ({ slug, event }) => {
           promo_markdown: (data?.event as { promo_markdown?: string | null })?.promo_markdown,
         }}
         brandingSettings={brandingSettings}
+        heroLogoVisible={data?.event?.hero_logo_visible !== false}
         headerStyle={data?.event?.header_style || theme.headerStyle}
         showLogout={true}
         onLogout={logout}