fix(admin): guard GET /api/users/{id} with @RequirePermission(ADMIN_USER)

Fixes IDOR: the endpoint was publicly accessible to any authenticated user.
Now requires ADMIN_USER permission, matching all other user management endpoints.

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

frontend/src/routes/admin/users/+page.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+</script>
+
+<div class="flex flex-1 items-center justify-center p-8">
+	<p class="text-sm text-ink-3">{m.admin_users_select_prompt()}</p>
+</div>