fix(purchase): 작업일정 삭제 시 관련 데이터 캐스케이드 삭제 (admin 전용)

- 삭제 권한을 admin 전용으로 변경 (requireAdmin)
- 트랜잭션으로 reports → checkins → safety_education → schedule 순서 삭제
- 프론트엔드: admin만 삭제 버튼 표시, 종속 데이터 삭제 경고 추가
- 404 처리 및 한국어 에러 메시지

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tkpurchase/api/models/scheduleModel.js

@@ -113,8 +113,25 @@ async function updateStatus(id, status) {
 }
 
 async function deleteSchedule(id) {
+  const schedule = await findById(id);
+  if (!schedule) return null;
+
   const db = getPool();
-  await db.query('DELETE FROM partner_schedules WHERE id = ?', [id]);
+  const conn = await db.getConnection();
+  try {
+    await conn.beginTransaction();
+    await conn.query('DELETE FROM partner_work_reports WHERE schedule_id = ?', [id]);
+    await conn.query('DELETE FROM partner_work_checkins WHERE schedule_id = ?', [id]);
+    await conn.query("DELETE FROM safety_education_reports WHERE target_type = 'partner_schedule' AND target_id = ?", [id]);
+    await conn.query('DELETE FROM partner_schedules WHERE id = ?', [id]);
+    await conn.commit();
+    return true;
+  } catch (err) {
+    await conn.rollback();
+    throw err;
+  } finally {
+    conn.release();
+  }
 }
 
 async function findActiveByCompany(companyId) {