[FEAT] Add nickname feature with fallback to name

[Copilot]

Implements nickname functionality for users with automatic fallback to the user's name when nickname is not set.

Changes Made

Database Schema

• Added nullable nickname column to the users table (max 50 characters)
• Maintains backward compatibility - existing users will have null nickname

API Endpoint

Added PATCH /api/v1/users/nickname endpoint:

// Request
{
  "nickname": "새로운닉네임"
}

// Response
{
  "message": "닉네임이 성공적으로 수정되었습니다.",
  "nickname": "새로운닉네임"
}

Core Logic

• User Entity: Added nickname field with updateNickname() and getDisplayName() methods
• Fallback Behavior: getDisplayName() returns nickname if available, otherwise returns name
• UserInfo DTO: Updated to use display name logic automatically
• Service Layer: Added UserService with transactional nickname update support

Validation

• Nickname is optional and limited to 50 characters
• Empty/whitespace-only nicknames fall back to name
• Input validation with proper error messages

Example Usage

// User with nickname
User user = User.builder()
    .name("김철수")
    .nickname("철수킹")
    .build();
    
user.getDisplayName(); // Returns "철수킹"

// User without nickname
User user2 = User.builder()
    .name("김영희")
    .nickname(null)
    .build();
    
user2.getDisplayName(); // Returns "김영희"

The implementation follows existing code patterns and includes comprehensive test coverage (16/16 tests passing).

Fixes #61.

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