Get best practices for developing applications with Spring Boot and Kotlin

# Spring Boot with Kotlin Best Practices

Your goal is to help me write high-quality, idiomatic Spring Boot applications using Kotlin.

## Project Setup & Structure

- **Build Tool:** Use Maven (`pom.xml`) or Gradle (`build.gradle`) with the Kotlin plugins (`kotlin-maven-plugin` or `org.jetbrains.kotlin.jvm`).
- **Kotlin Plugins:** For JPA, enable the `kotlin-jpa` plugin to automatically make entity classes `open` without boilerplate.
- **Starters:** Use Spring Boot starters (e.g., `spring-boot-starter-web`, `spring-boot-starter-data-jpa`) as usual.
- **Package Structure:** Organize code by feature/domain (e.g., `com.example.app.order`, `com.example.app.user`) rather than by layer.

## Dependency Injection & Components

- **Primary Constructors:** Always use the primary constructor for required dependency injection. It's the most idiomatic and concise approach in Kotlin.
- **Immutability:** Declare dependencies as `private val` in the primary constructor. Prefer `val` over `var` everywhere to promote immutability.
- **Component Stereotypes:** Use `@Service`, `@Repository`, and `@RestController` annotations just as you would in Java.

## Configuration

- **Externalized Configuration:** Use `application.yml` for its readability and hierarchical structure.
- **Type-Safe Properties:** Use `@ConfigurationProperties` with `data class` to create immutable, type-safe configuration objects.
- **Profiles:** Use Spring Profiles (`application-dev.yml`, `application-prod.yml`) to manage environment-specific configurations.
- **Secrets Management:** Never hardcode secrets. Use environment variables or a dedicated secret management tool like HashiCorp Vault or AWS Secrets Manager.

## Web Layer (Controllers)