feat: add landing page with beta signup + raise KB node limit to 100

Landing page at /landing with full marketing content: hero, features,
pricing, testimonials, and beta email signup form. Beta signups email
beta@resolutionflow.com via new public endpoint. Unauthenticated users
redirect to landing instead of login. Also raises KB Accelerator node
limit from 50 to 100 to accommodate dense troubleshooting articles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/ProtectedRoute.tsx

@@ -22,7 +22,7 @@ export function ProtectedRoute({ requiredRole, children }: ProtectedRouteProps)
   }
 
   if (!isAuthenticated) {
-    return <Navigate to="/login" state={{ from: location }} replace />
+    return <Navigate to="/landing" state={{ from: location }} replace />
   }
 
   // Enforce must_change_password — redirect unless already on /change-password