feat: 과목 삭제 기능 — 연관 데이터 트랜잭션 정리

- backend: subjects.service.remove()를 트랜잭션으로 변경
  (reviewSchedule → studyLog → skillSnapshot → subject 순서 삭제)
- frontend: 과목 카드에 삭제 버튼 + 확인 다이얼로그 추가

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/subjects/subjects.service.ts

@@ -37,7 +37,48 @@ export class SubjectsService {
   async remove(userId: number, id: number) {
     const owned = await this.prisma.subject.findFirst({ where: { id, userId } });
     if (!owned) throw new NotFoundException();
-    await this.prisma.subject.delete({ where: { id } });
-    return { ok: true };
+
+    return this.prisma.$transaction(async (tx) => {
+      // 1. 이 과목에 속한 studyLog ID 목록
+      const logs = await tx.studyLog.findMany({
+        where: { userId, subjectId: id },
+        select: { id: true },
+      });
+      const logIds = logs.map((l) => l.id);
+
+      // 2. 연관 reviewSchedule 삭제
+      if (logIds.length > 0) {
+        await tx.reviewSchedule.deleteMany({
+          where: { studyLogId: { in: logIds } },
+        });
+      }
+
+      // 3. studyLog 삭제
+      await tx.studyLog.deleteMany({
+        where: { userId, subjectId: id },
+      });
+
+      // 4. skillSnapshot 삭제 (태그 경유)
+      const tagIds = await tx.tag.findMany({
+        where: { subjectId: id },
+        select: { id: true },
+      });
+      if (tagIds.length > 0) {
+        await tx.skillSnapshot.deleteMany({
+          where: { userId, tagId: { in: tagIds.map((t) => t.id) } },
+        });
+      }
+
+      // 5. 과목 삭제 (tags는 onDelete: Cascade로 자동 삭제)
+      await tx.subject.delete({ where: { id } });
+
+      return {
+        ok: true,
+        deleted: {
+          studyLogs: logIds.length,
+          tags: tagIds.length,
+        },
+      };
+    });
   }
 }

frontend/src/app/subjects/page.tsx

@@ -101,6 +101,9 @@ function SubjectsBody() {
     subjectId: number;
   } | null>(null);
 
+  const [deleteSubjectCandidate, setDeleteSubjectCandidate] =
+    useState<SubjectWithTags | null>(null);
+
   const loadSubjects = useCallback(async () => {
     setLoading(true);
     setLoadError(null);
@@ -340,6 +343,27 @@ function SubjectsBody() {
     }
   };
 
+  const confirmDeleteSubject = async () => {
+    if (!deleteSubjectCandidate) return;
+    const subject = deleteSubjectCandidate;
+    setDeleteSubjectCandidate(null);
+
+    try {
+      await api.delete(`/subjects/${subject.id}`);
+      setSubjects((prev) => prev?.filter((s) => s.id !== subject.id) ?? null);
+      setSelectedTab('all');
+      showToast({
+        variant: 'success',
+        message: `'${subject.name}' 과목을 삭제했어요.`,
+      });
+    } catch {
+      showToast({
+        variant: 'danger',
+        message: '과목 삭제에 실패했어요.',
+      });
+    }
+  };
+
   if (loading) {
     return (
       <PageWrap>
@@ -374,6 +398,17 @@ function SubjectsBody() {
         onCancel={() => setDeleteCandidate(null)}
       />
 
+      <ConfirmDialog
+        open={deleteSubjectCandidate !== null}
+        title="과목 삭제"
+        body={`'${deleteSubjectCandidate?.name}' 과목과 하위 태그 ${deleteSubjectCandidate?.tags.length ?? 0}개, 관련 학습 기록이 모두 삭제돼요. 되돌릴 수 없어요.`}
+        confirmLabel="과목 삭제"
+        cancelLabel="취소"
+        tone="danger"
+        onConfirm={() => void confirmDeleteSubject()}
+        onCancel={() => setDeleteSubjectCandidate(null)}
+      />
+
       <SubjectModal
         open={subjectModalOpen}
         mode={subjectDraft.id === null ? 'create' : 'edit'}
@@ -473,6 +508,13 @@ function SubjectsBody() {
                       <Icon name="plus" size={16} weight="bold" />
                       태그 추가
                     </HeaderActionButton>
+                    <DangerActionButton
+                      type="button"
+                      onClick={() => setDeleteSubjectCandidate(subject)}
+                    >
+                      <Icon name="x" size={16} />
+                      삭제
+                    </DangerActionButton>
                   </SubjectHeaderActions>
                 </SubjectCardHeader>
 
@@ -1197,6 +1239,27 @@ const HeaderActionButton = styled.button`
   }
 `;
 
+const DangerActionButton = styled.button`
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+  min-height: 38px;
+  padding: 0 12px;
+  border: 1px solid transparent;
+  border-radius: ${theme.radius.md};
+  background: transparent;
+  color: ${theme.color.textMute};
+  font-size: 13px;
+  font-weight: 600;
+  transition: all 0.16s ease;
+
+  &:hover {
+    color: ${theme.color.danger};
+    border-color: rgba(239, 68, 68, 0.35);
+    background: rgba(239, 68, 68, 0.08);
+  }
+`;
+
 const TagComposer = styled.div`
   display: grid;
   grid-template-columns: minmax(0, 1fr) auto;