feat: 쪽지 전체 조회 구현

src/main/java/com/cake/pop/domain/letter/controller/LetterController.java

@@ -1,13 +1,16 @@
 package com.cake.pop.domain.letter.controller;
 
 import com.cake.pop.domain.letter.dto.request.CreateLetterRequest;
+import com.cake.pop.domain.letter.dto.response.GetLettersResponse;
 import com.cake.pop.domain.letter.service.LetterService;
 import jakarta.validation.Valid;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController
@@ -18,9 +21,14 @@ public class LetterController {
     private final LetterService letterService;
 
     @PostMapping
-    public ResponseEntity<Void> createLetter(@RequestBody @Valid CreateLetterRequest request){
+    public ResponseEntity<Void> createLetter(@RequestBody @Valid CreateLetterRequest request) {
         letterService.create(request);
-
         return ResponseEntity.ok().build();
     }
+
+    @GetMapping
+    public ResponseEntity<GetLettersResponse> getLetters(@RequestParam("region") String region) {
+        GetLettersResponse response = letterService.getLetters(region);
+        return ResponseEntity.ok(response);
+    }
 }

src/main/java/com/cake/pop/domain/letter/dto/response/GetLettersResponse.java

@@ -1,4 +1,9 @@
 package com.cake.pop.domain.letter.dto.response;
 
-public record GetLettersResponse() {
+import java.util.List;
+
+public record GetLettersResponse(
+    String region,
+    List<SimpleLetterDto> letters
+) {
 }

src/main/java/com/cake/pop/domain/letter/dto/response/SimpleLetterDto.java

@@ -0,0 +1,12 @@
+package com.cake.pop.domain.letter.dto.response;
+
+import com.fasterxml.jackson.annotation.JsonFormat;
+import java.time.LocalDate;
+
+public record SimpleLetterDto(
+        Long letterId,
+        String content,
+        @JsonFormat(pattern = "yy.MM.dd")
+        LocalDate createdAt
+) {
+}

src/main/java/com/cake/pop/domain/letter/repository/LetterRepository.java

@@ -2,13 +2,17 @@
 
 import com.cake.pop.domain.letter.exception.LetterErrorCode;
 import com.cake.pop.entity.Letter;
+import com.cake.pop.entity.Mailbox;
 import com.cake.pop.global.exception.RestApiException;
+import java.util.List;
 import org.springframework.data.jpa.repository.JpaRepository;
 import org.springframework.stereotype.Repository;
 
 @Repository
 public interface LetterRepository extends JpaRepository<Letter, Long> {
 
+    List<Letter> findByMailbox(Mailbox mailbox);
+
     default Letter getById(Long id) {
         return findById(id).orElseThrow(() -> new RestApiException(LetterErrorCode.LETTER_NOT_FOUND));
     }

src/main/java/com/cake/pop/domain/letter/service/LetterService.java

@@ -1,10 +1,14 @@
 package com.cake.pop.domain.letter.service;
 
 import com.cake.pop.domain.letter.dto.request.CreateLetterRequest;
+import com.cake.pop.domain.letter.dto.response.GetLettersResponse;
+import com.cake.pop.domain.letter.dto.response.SimpleLetterDto;
 import com.cake.pop.domain.letter.repository.LetterRepository;
 import com.cake.pop.domain.letter.repository.MailboxRepository;
 import com.cake.pop.entity.Letter;
 import com.cake.pop.entity.Mailbox;
+import com.cake.pop.entity.enums.Region;
+import java.util.List;
 import lombok.RequiredArgsConstructor;
 import org.springframework.stereotype.Service;
 
@@ -20,4 +24,20 @@ public void create(CreateLetterRequest request) {
         Letter letter = Letter.of(request.content(), findMailbox, request.imageUrl());
         letterRepository.save(letter);
     }
+
+    public GetLettersResponse getLetters(String region){
+        Mailbox findMailbox = mailboxRepository.getFirstByRegion(Region.fromKoreanName(region));
+
+        List<Letter> letters = letterRepository.findByMailbox(findMailbox);
+
+        List<SimpleLetterDto> letterDtos = letters.stream()
+                .map(letter -> new SimpleLetterDto(
+                        letter.getId(),
+                        letter.getContent(),
+                        letter.getCreatedAt().toLocalDate()
+                ))
+                .toList();
+
+        return new GetLettersResponse(region, letterDtos);
+    }
 }