feat: 쪽지 기능 뼈대코드 설계 및 구현

backend/src/main/java/com/now/naaga/letter/domain/Letter.java

@@ -0,0 +1,87 @@
+package com.now.naaga.letter.domain;
+
+import com.now.naaga.common.domain.BaseEntity;
+import com.now.naaga.place.domain.Position;
+import com.now.naaga.player.domain.Player;
+import jakarta.persistence.*;
+
+import java.util.Objects;
+
+@Entity
+public class Letter extends BaseEntity {
+
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    @Id
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "player_id")
+    private Player registeredPlayer;
+
+    @Embedded
+    private Position position;
+
+    private String message;
+
+    public Letter() {
+    }
+
+    public Letter(final Player registeredPlayer,
+                  final Position position,
+                  final String message) {
+        this(null, registeredPlayer, position, message);
+    }
+
+    public Letter(final Long id,
+                  final Player registeredPlayer,
+                  final Position position,
+                  final String message) {
+        this.id = id;
+        this.registeredPlayer = registeredPlayer;
+        this.position = position;
+        this.message = message;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public Player getRegisteredPlayer() {
+        return registeredPlayer;
+    }
+
+    public Position getPosition() {
+        return position;
+    }
+
+    public String getMessage() {
+        return message;
+    }
+
+    @Override
+    public boolean equals(final Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        Letter letter = (Letter) o;
+        return Objects.equals(id, letter.id);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(id);
+    }
+
+    @Override
+    public String toString() {
+        return "Letter{" +
+                "id=" + id +
+                ", playerId=" + registeredPlayer.getId() +
+                ", position=" + position +
+                ", message='" + message +
+                '}';
+    }
+}

backend/src/main/java/com/now/naaga/letter/presentation/LetterController.java

@@ -0,0 +1,9 @@
+package com.now.naaga.letter.presentation;
+
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+@RequestMapping("/letters")
+@RestController
+public class LetterController {
+}

backend/src/main/java/com/now/naaga/letter/repository/LetterRepository.java

@@ -0,0 +1,7 @@
+package com.now.naaga.letter.repository;
+
+import com.now.naaga.letter.domain.Letter;
+import org.springframework.data.jpa.repository.JpaRepository;
+
+public interface LetterRepository extends JpaRepository<Letter, Long> {
+}

backend/src/test/java/com/now/naaga/common/builder/LetterBuilder.java

@@ -0,0 +1,62 @@
+package com.now.naaga.common.builder;
+
+import com.now.naaga.letter.domain.Letter;
+import com.now.naaga.letter.repository.LetterRepository;
+import com.now.naaga.place.domain.Position;
+import com.now.naaga.player.domain.Player;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.stereotype.Component;
+
+import java.util.Optional;
+
+import static com.now.naaga.common.fixture.LetterFixture.MESSAGE;
+import static com.now.naaga.common.fixture.PositionFixture.잠실역_교보문고_좌표;
+
+@Component
+public class LetterBuilder {
+
+    @Autowired
+    private LetterRepository letterRepository;
+
+    @Autowired
+    private PlayerBuilder playerBuilder;
+
+    private Optional<Player> registeredPlayer;
+
+    private Position position;
+
+    private String message;
+
+    public LetterBuilder init() {
+        this.registeredPlayer = Optional.empty();
+        this.position = 잠실역_교보문고_좌표;
+        this.message = MESSAGE;
+        return this;
+    }
+
+    public LetterBuilder registeredPlayer(final Player persistedPlayer) {
+        this.registeredPlayer = Optional.ofNullable(persistedPlayer);
+        return this;
+    }
+
+    public LetterBuilder position(final Position position) {
+        this.position = position;
+        return this;
+    }
+
+    public LetterBuilder message(final String message) {
+        this.message = message;
+        return this;
+    }
+
+    public Letter build() {
+        final Player persistedPlayer = registeredPlayer.orElseGet(this::getPersistedPlayer);
+        final Letter letter = new Letter(persistedPlayer, position, message);
+        return letterRepository.save(letter);
+    }
+
+    private Player getPersistedPlayer() {
+        return playerBuilder.init()
+                .build();
+    }
+}

backend/src/test/java/com/now/naaga/common/fixture/LetterFixture.java

@@ -0,0 +1,15 @@
+package com.now.naaga.common.fixture;
+
+import com.now.naaga.letter.domain.Letter;
+
+import static com.now.naaga.common.fixture.PlayerFixture.PLAYER;
+import static com.now.naaga.common.fixture.PositionFixture.잠실_루터회관_정문_좌표;
+
+public class LetterFixture {
+
+    public static final String MESSAGE = "안녕하세요. 나아가 개발자들 입니다.";
+
+    public static Letter LETTER() {
+        return new Letter(PLAYER(), 잠실_루터회관_정문_좌표, MESSAGE);
+    }
+}

backend/src/test/java/com/now/naaga/common/fixture/PositionFixture.java

@@ -1,6 +1,7 @@
 package com.now.naaga.common.fixture;
 
 import com.now.naaga.place.domain.Position;
+
 import java.math.BigDecimal;
 
 public class PositionFixture {