feat(auth): Supabase Auth email/password + middleware + login [auth]

Protege /admin/* con Supabase Auth. Tabla user_roles con custom claims
(app_role, org_id) en JWT. Middleware Next.js redirige a /login sin sesión.
Página /login con email/password. /admin/responses migrado de service_role
a sesión de usuario autenticado.

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

app/admin/sign-out-button.tsx

@@ -0,0 +1,24 @@
+'use client'
+
+import { createBrowserClient } from '@supabase/ssr'
+import { useRouter } from 'next/navigation'
+
+export function SignOutButton() {
+  const router = useRouter()
+
+  const supabase = createBrowserClient(
+    process.env.NEXT_PUBLIC_SUPABASE_URL!,
+    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!
+  )
+
+  async function handleSignOut() {
+    await supabase.auth.signOut()
+    router.push('/login')
+  }
+
+  return (
+    <button type="button" onClick={handleSignOut} className="admin-signout">
+      Cerrar sesión
+    </button>
+  )
+}