chore: add mandatory test concept and smoke-test workflow

backend/scripts/smoke-test.mjs

@@ -0,0 +1,20 @@
+import { spawn } from 'node:child_process';
+
+const run = (cmd, args, opts = {}) =>
+  new Promise((resolve, reject) => {
+    const p = spawn(cmd, args, { stdio: 'inherit', ...opts });
+    p.on('exit', (code) => (code === 0 ? resolve() : reject(new Error(`${cmd} exited with ${code}`))));
+  });
+
+const main = async () => {
+  await run('bash', ['-lc', 'find src -name "*.js" -print0 | xargs -0 -n1 node --check'], {
+    cwd: process.cwd(),
+  });
+
+  console.log('Smoke test passed: syntax checks OK');
+};
+
+main().catch((err) => {
+  console.error('Smoke test failed:', err.message);
+  process.exit(1);
+});