feat: 메시지 미리보기 API 구현 (#120)

- POST /v1/in/message/preview 엔드포인트 추가
- MessageService: 메시지 조회 → 변수 치환 → 미리보기 데이터 반환
- IMessageService 인터페이스 정의 (향후 CRUD 확장용)

Closes #120

SPMS.API/Controllers/MessageController.cs

@@ -12,10 +12,12 @@ namespace SPMS.API.Controllers;
 public class MessageController : ControllerBase
 {
     private readonly IMessageValidationService _validationService;
+    private readonly IMessageService _messageService;
 
-    public MessageController(IMessageValidationService validationService)
+    public MessageController(IMessageValidationService validationService, IMessageService messageService)
     {
         _validationService = validationService;
+        _messageService = messageService;
     }
 
     [HttpPost("validate")]
@@ -26,6 +28,15 @@ public class MessageController : ControllerBase
         return Ok(ApiResponse<MessageValidationResultDto>.Success(result));
     }
 
+    [HttpPost("preview")]
+    [SwaggerOperation(Summary = "메시지 미리보기", Description = "메시지 템플릿에 변수를 치환하여 미리보기를 생성합니다.")]
+    public async Task<IActionResult> PreviewAsync([FromBody] MessagePreviewRequestDto request)
+    {
+        var serviceId = GetServiceId();
+        var result = await _messageService.PreviewAsync(serviceId, request);
+        return Ok(ApiResponse<MessagePreviewResponseDto>.Success(result));
+    }
+
     private long GetServiceId()
     {
         if (HttpContext.Items.TryGetValue("ServiceId", out var serviceIdObj) && serviceIdObj is long serviceId)

SPMS.Application/DTOs/Message/MessagePreviewRequestDto.cs

@@ -0,0 +1,11 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace SPMS.Application.DTOs.Message;
+
+public class MessagePreviewRequestDto
+{
+    [Required]
+    public string MessageCode { get; set; } = string.Empty;
+
+    public Dictionary<string, string>? Variables { get; set; }
+}

SPMS.Application/DTOs/Message/MessagePreviewResponseDto.cs

@@ -0,0 +1,9 @@
+namespace SPMS.Application.DTOs.Message;
+
+public class MessagePreviewResponseDto
+{
+    public string Title { get; set; } = string.Empty;
+    public string Body { get; set; } = string.Empty;
+    public string? ImageUrl { get; set; }
+    public string? LinkUrl { get; set; }
+}

SPMS.Application/DependencyInjection.cs

@@ -20,6 +20,7 @@ public static class DependencyInjection
         services.AddScoped<IFileService, FileService>();
         services.AddScoped<IPushService, PushService>();
         services.AddSingleton<IMessageValidationService, MessageValidationService>();
+        services.AddScoped<IMessageService, MessageService>();
 
         return services;
     }

SPMS.Application/Interfaces/IMessageService.cs

@@ -0,0 +1,8 @@
+using SPMS.Application.DTOs.Message;
+
+namespace SPMS.Application.Interfaces;
+
+public interface IMessageService
+{
+    Task<MessagePreviewResponseDto> PreviewAsync(long serviceId, MessagePreviewRequestDto request);
+}

SPMS.Application/Services/MessageService.cs

@@ -0,0 +1,48 @@
+using SPMS.Application.DTOs.Message;
+using SPMS.Application.Interfaces;
+using SPMS.Domain.Common;
+using SPMS.Domain.Exceptions;
+using SPMS.Domain.Interfaces;
+
+namespace SPMS.Application.Services;
+
+public class MessageService : IMessageService
+{
+    private readonly IMessageRepository _messageRepository;
+
+    public MessageService(IMessageRepository messageRepository)
+    {
+        _messageRepository = messageRepository;
+    }
+
+    public async Task<MessagePreviewResponseDto> PreviewAsync(long serviceId, MessagePreviewRequestDto request)
+    {
+        var message = await _messageRepository.GetByMessageCodeAndServiceAsync(request.MessageCode, serviceId);
+        if (message == null)
+            throw new SpmsException(ErrorCodes.MessageNotFound, "존재하지 않는 메시지 코드입니다.", 404);
+
+        var title = ApplyVariables(message.Title, request.Variables);
+        var body = ApplyVariables(message.Body, request.Variables);
+
+        return new MessagePreviewResponseDto
+        {
+            Title = title,
+            Body = body,
+            ImageUrl = message.ImageUrl,
+            LinkUrl = message.LinkUrl
+        };
+    }
+
+    private static string ApplyVariables(string template, Dictionary<string, string>? variables)
+    {
+        if (variables == null || variables.Count == 0)
+            return template;
+
+        var result = template;
+        foreach (var (key, value) in variables)
+        {
+            result = result.Replace($"{{{{{key}}}}}", value);
+        }
+        return result;
+    }
+}