[feat]: 블랙리스트 엔터티 구현

src/main/java/com/mulmi/backend/domain/blacklist/entity/Blacklist.java

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+package com.mulmi.backend.domain.blacklist.entity;
+
+import com.mulmi.backend.domain.user.entity.User;
+import com.mulmi.backend.global.common.BaseEntity;
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.FetchType;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.Index;
+import jakarta.persistence.JoinColumn;
+import jakarta.persistence.ManyToOne;
+import jakarta.persistence.Table;
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+
+import java.time.LocalDate;
+
+@Entity
+@Getter
+@Builder
+@NoArgsConstructor(access = AccessLevel.PROTECTED)
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Table(
+        name = "blacklists",
+        indexes = @Index(
+                name = "idx_blacklists_user_period",
+                columnList = "user_id,start_date,end_date"
+        )
+)
+public class Blacklist extends BaseEntity {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id", nullable = false)
+    private User user;
+
+    // Rental 엔터티 구현 후 연관관계로 전환한다.
+    @Column(name = "rental_id")
+    private Long rentalId;
+
+    @Column(name = "start_date", nullable = false)
+    private LocalDate startDate;
+
+    @Column(name = "end_date", nullable = false)
+    private LocalDate endDate;
+
+    @Column(nullable = false, columnDefinition = "TEXT")
+    private String reason;
+
+    public void updatePeriodAndReason(
+            LocalDate startDate,
+            LocalDate endDate,
+            String reason
+    ) {
+        this.startDate = startDate;
+        this.endDate = endDate;
+        this.reason = reason;
+    }
+
+    public boolean overlaps(LocalDate otherStartDate, LocalDate otherEndDate) {
+        return !endDate.isBefore(otherStartDate)
+                && !startDate.isAfter(otherEndDate);
+    }
+}

src/test/java/com/mulmi/backend/domain/blacklist/entity/BlacklistTest.java

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+package com.mulmi.backend.domain.blacklist.entity;
+
+import com.mulmi.backend.domain.user.entity.User;
+import com.mulmi.backend.domain.user.enums.UserRole;
+import com.mulmi.backend.domain.user.enums.UserStatus;
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class BlacklistTest {
+
+    @Test
+    void updatePeriodAndReasonChangesBlacklistDetails() {
+        Blacklist blacklist = createBlacklist(
+                LocalDate.of(2026, 9, 1),
+                LocalDate.of(2026, 9, 30)
+        );
+
+        blacklist.updatePeriodAndReason(
+                LocalDate.of(2026, 10, 1),
+                LocalDate.of(2026, 10, 15),
+                "반납 지연"
+        );
+
+        assertThat(blacklist.getStartDate()).isEqualTo(LocalDate.of(2026, 10, 1));
+        assertThat(blacklist.getEndDate()).isEqualTo(LocalDate.of(2026, 10, 15));
+        assertThat(blacklist.getReason()).isEqualTo("반납 지연");
+    }
+
+    @Test
+    void overlapsIncludesBoundaryDates() {
+        Blacklist blacklist = createBlacklist(
+                LocalDate.of(2026, 9, 1),
+                LocalDate.of(2026, 9, 30)
+        );
+
+        assertThat(blacklist.overlaps(
+                LocalDate.of(2026, 9, 30),
+                LocalDate.of(2026, 10, 10)
+        )).isTrue();
+        assertThat(blacklist.overlaps(
+                LocalDate.of(2026, 10, 1),
+                LocalDate.of(2026, 10, 10)
+        )).isFalse();
+    }
+
+    private Blacklist createBlacklist(LocalDate startDate, LocalDate endDate) {
+        User user = User.builder()
+                .id(1L)
+                .loginId("C123456")
+                .studentId("C123456")
+                .password("encoded-password")
+                .name("홍길동")
+                .email("hong@example.com")
+                .phoneNumber("01012345678")
+                .college("공과대학")
+                .department("컴퓨터공학과")
+                .role(UserRole.STUDENT)
+                .status(UserStatus.ACTIVE)
+                .build();
+
+        return Blacklist.builder()
+                .id(1L)
+                .user(user)
+                .rentalId(101L)
+                .startDate(startDate)
+                .endDate(endDate)
+                .reason("장기 연체")
+                .build();
+    }
+}