feat(backend): add foundation services and sql idempotency

backend/command-api/src/middleware/idempotency.js

@@ -0,0 +1,10 @@
+import { AppError } from '../lib/errors.js';
+
+export function requireIdempotencyKey(req, _res, next) {
+  const idempotencyKey = req.get('Idempotency-Key');
+  if (!idempotencyKey) {
+    return next(new AppError('MISSING_IDEMPOTENCY_KEY', 'Missing Idempotency-Key header', 400));
+  }
+  req.idempotencyKey = idempotencyKey;
+  return next();
+}