feat(enterprise): add custom branding system — logo, accent color, company name

- Add branding_logo_url, branding_primary_color, branding_company_name columns to Account model
- Add Alembic migration (58e3f27f3e8f) for branding and SSO columns
- Add GET/PATCH /accounts/me/branding endpoints (owner-only for PATCH)
- Add BrandingSettingsPage with logo URL input, color picker, preview section
- Add /account/branding route (ProtectedRoute owner) in router.tsx
- Add Branding link card in AccountSettingsPage

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/router.tsx

@@ -76,6 +76,7 @@ const TeamCategoriesPage = lazy(() => import('@/pages/account/TeamCategoriesPage
 const TargetListsPage = lazy(() => import('@/pages/account/TargetListsPage'))
 const ChatRetentionSettingsPage = lazy(() => import('@/pages/account/ChatRetentionSettingsPage'))
 const IntegrationsPage = lazy(() => import('@/pages/account/IntegrationsPage'))
+const BrandingSettingsPage = lazy(() => import('@/pages/account/BrandingSettingsPage'))
 
 /** Wraps a lazy-loaded page with Suspense + ErrorBoundary */
 function page(Component: React.LazyExoticComponent<React.ComponentType>) {
@@ -252,6 +253,14 @@ export const router = sentryCreateBrowserRouter([
               </ProtectedRoute>
             ),
           },
+          {
+            path: 'branding',
+            element: (
+              <ProtectedRoute requiredRole="owner">
+                {page(BrandingSettingsPage)}
+              </ProtectedRoute>
+            ),
+          },
         ],
       },
     ],