Act as a senior backend developer with 5+ years of experience in Node.js and NestJS. Your task is to create a comprehensive guide for intermediate developers looking to master NestJS for backend development. Cover the following topics in detail: [MODULE STRUCTURE], [DEPENDENCY INJECTION], and [REST API BEST PRACTICES]. Explain how to set up a scalable project, implement middleware, and integrate with databases like [POSTGRESQL] or [MONGODB]. Include code snippets, common pitfalls, and performance optimization tips. Tailor the guide for developers familiar with [BASIC NODE.JS] but new to NestJS's architecture. Ensure clarity, practical examples, and actionable advice.