[MR Train 4/6] Refactor: Add REST API endpoints

MR Train Overview

This is MR 4 of 6 in a stacked MR train.

Train Structure

1. [MR 1/6] Foundation (!2 (merged))
2. [MR 2/6] Database (!3 (merged))
3. [MR 3/6] Service layer (!4 (merged))
4. [MR 4/6] API endpoints ← YOU ARE HERE
5. [MR 5/6] Integration tests
6. [MR 6/6] Performance optimizations

This MR: REST API

Changes

• OrderController with full CRUD operations
• OpenAPI/Swagger annotations
• Proper HTTP status codes

⚠️ Depends on: !4 (merged) (MR 3/6)