refactor: 인증/공통 도메인 마이그레이션 및 Kotlin 마이그레이션 최종 마무리

[jwnnoh]

1. 무슨 이유로 코드를 변경했나요?

인증 도메인, 이미지 컨트롤러 및 기타 공통 패키지 마이그레이션을 진행했습니다.
마이그레이션이 마무리 됨에 따라 JVM 21로 빌드 구현체 또한 마이그레이션 완료하였습니다.

## 2. 어떤 위험이나 장애를 발견했나요? - 특이 사항 없음.

3. 관련 스크린샷을 첨부해주세요.

4. 완료 사항

• 인증/공통 도메인 마이그레이션
• 빌드 구현체 마이그레이션
• 리드미 수정

5. 추가 사항

• build.gradle.kts 설정을 기준으로 문서를 업데이트했습니다.

[Copilot]

Pull request overview

This PR completes the Kotlin migration by converting authentication and shared domain components from Java to Kotlin, along with upgrading the JVM target from version 17 to 21. The migration includes updating repository methods to use Kotlin's nullable types instead of Java's Optional, simplifying test mocks, and migrating configuration files.

Key changes include:

• Complete migration of authentication domain (AuthService, AuthDetails, AuthController) from Java to Kotlin
• Repository interfaces updated to return nullable types instead of Optional
• JVM target upgraded from 17 to 21 in build configuration
• Documentation updates reflecting the migration completion

Reviewed changes

Copilot reviewed 41 out of 42 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/main/kotlin/land/leets/LeetsApplication.kt	Migrated main application class from Java to Kotlin with idiomatic syntax
src/main/kotlin/land/leets/domain/auth/*	Migrated authentication services, controllers, and DTOs to Kotlin
src/main/kotlin/land/leets/domain/shared/BaseTimeEntity.kt	Migrated base entity with audit timestamps to Kotlin
src/main/kotlin/land/leets/domain/admin/domain/repository/AdminRepository.kt	Updated repository to return nullable Admin instead of Optional
src/main/kotlin/land/leets/domain/user/domain/repository/UserRepository.kt	Updated repository to return nullable User instead of Optional
src/main/kotlin/land/leets/global/advice/ExceptionHandleAdvice.kt	Migrated exception handler with package name fix
src/main/kotlin/land/leets/global/advice/ResponseAdvice.kt	Migrated response wrapper advice to Kotlin
src/main/kotlin/land/leets/global/config/*	Migrated configuration classes to Kotlin
src/main/kotlin/land/leets/global/swagger/SwaggerConfig.kt	Migrated Swagger configuration to Kotlin
src/test/kotlin/land/leets/domain/admin/usecase/AdminLoginImplTest.kt	Updated tests to use nullable returns instead of Optional
src/test/kotlin/land/leets/domain/application/usecase/*Test.kt	Simplified tests by replacing mocked AuthDetails with direct instantiation
src/test/kotlin/land/leets/LeetsApplicationTests.kt	Migrated empty Spring Boot test class to Kotlin
build.gradle.kts	Updated JVM target from 17 to 21 and reorganized compiler options
README.md	Updated documentation with new tech stack badges and improved formatting

Comments suppressed due to low confidence (1)

src/main/kotlin/land/leets/global/advice/ExceptionHandleAdvice.kt:44

• The explicit Companion usage is unnecessary in Kotlin. When calling static methods on companion objects, you can call them directly without the .Companion qualifier. This applies to all three usages of ErrorResponse.Companion.from(), ErrorResponse.Companion.of(), and String.Companion.format().

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[yechan-kim]

고생 많으셨습니다! 정완님~

[yechan-kim]

👍