feat(settings): add admin settings API routes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/routes/api/admin/settings/[key].ts

@@ -0,0 +1,12 @@
+import type { ActionFunction } from '@remix-run/node';
+import { settingsService } from '~/lib/settings.server';
+import { requireAdmin } from '~/lib/auth.server';
+
+export const action: ActionFunction = async ({ request, params }) => {
+  await requireAdmin(request);
+  const key = params.key as string;
+  const body = await request.json();
+  if (!key) return new Response('Missing key', { status: 400 });
+  await settingsService.set(key, body.value, 'admin');
+  return new Response(null, { status: 204 });
+};