feat(tkpurchase): 체크인 worker_names 배열 저장 + 구매팀 체크인 관리 기능

- doCheckIn()에서 worker_names를 콤마 split 배열로 전송 (DB에 JSON 배열로 저장)
- 구매팀 일정 페이지에 체크인 조회/수정/삭제 모달 추가
- DELETE /checkins/:id endpoint + 트랜잭션 삭제 (reports cascade)
- PUT /checkins/:id에 requirePage 권한 guard 추가

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

tkpurchase/api/models/checkinModel.js

@@ -78,4 +78,32 @@ async function countActive() {
   return rows[0].cnt;
 }
 
-module.exports = { findBySchedule, findById, findTodayByCompany, checkIn, checkOut, update, resetCheckout, countActive };
+async function deleteCheckin(id) {
+  const checkin = await findById(id);
+  if (!checkin) return false;
+
+  const db = getPool();
+  const conn = await db.getConnection();
+  try {
+    await conn.beginTransaction();
+    await conn.query('DELETE FROM partner_work_reports WHERE checkin_id = ?', [id]);
+    await conn.query('DELETE FROM partner_work_checkins WHERE id = ?', [id]);
+    const [remaining] = await conn.query(
+      'SELECT COUNT(*) AS cnt FROM partner_work_checkins WHERE schedule_id = ?',
+      [checkin.schedule_id]);
+    if (remaining[0].cnt === 0) {
+      await conn.query(
+        "UPDATE partner_schedules SET status = 'scheduled' WHERE id = ? AND status = 'in_progress'",
+        [checkin.schedule_id]);
+    }
+    await conn.commit();
+    return true;
+  } catch (err) {
+    await conn.rollback();
+    throw err;
+  } finally {
+    conn.release();
+  }
+}
+
+module.exports = { findBySchedule, findById, findTodayByCompany, checkIn, checkOut, update, resetCheckout, countActive, deleteCheckin };