feat(settings): add admin settings API routes

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

app/routes/api/admin/__tests__/settings.api.test.ts

@@ -0,0 +1,8 @@
+import { settingsService } from '../../../lib/settings.server';
+
+test('settings API helper behavior', async () => {
+  const key = 'api_test_' + Date.now();
+  await settingsService.set(key, { foo: 'bar' }, 'test');
+  const v = await settingsService.get(key);
+  expect(v).toEqual({ foo: 'bar' });
+});