feat(purchase): 구매 취소/반품 + 입고일 기준 월별 분석

- 상태 추가: cancelled(구매취소), returned(반품)
- API: PUT /:id/cancel, /:id/return, /:id/revert-cancel
- 데스크탑: 구매완료→취소 버튼, 입고완료→반품 버튼, 취소→되돌리기
- 분석 페이지: 구매일/입고일 기준 전환 토글, 입고일 기준 월간 분류 집계 + 입고 목록
- Settlement API: GET /received-summary, /received-list (입고일 기준)

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

system1-factory/api/models/purchaseRequestModel.js

@@ -233,6 +233,43 @@ const PurchaseRequestModel = {
       [batchId]
     );
     return rows.map(r => r.requester_id);
+  },
+
+  // 구매 취소 (purchased → pending 복원, batch에서도 제거)
+  async cancelPurchase(requestId, { cancelledBy, cancelReason }) {
+    const db = await getDb();
+    await db.query(
+      `UPDATE purchase_requests
+       SET status = 'cancelled', cancelled_at = NOW(), cancelled_by = ?, cancel_reason = ?,
+           batch_id = NULL
+       WHERE request_id = ? AND status = 'purchased'`,
+      [cancelledBy, cancelReason || null, requestId]
+    );
+    return this.getById(requestId);
+  },
+
+  // 반품 (received → returned)
+  async returnItem(requestId, { cancelledBy, cancelReason }) {
+    const db = await getDb();
+    await db.query(
+      `UPDATE purchase_requests
+       SET status = 'returned', cancelled_at = NOW(), cancelled_by = ?, cancel_reason = ?
+       WHERE request_id = ? AND status = 'received'`,
+      [cancelledBy, cancelReason || null, requestId]
+    );
+    return this.getById(requestId);
+  },
+
+  // 취소/반품에서 원래 상태로 되돌리기
+  async revertCancel(requestId) {
+    const db = await getDb();
+    await db.query(
+      `UPDATE purchase_requests
+       SET status = 'pending', cancelled_at = NULL, cancelled_by = NULL, cancel_reason = NULL
+       WHERE request_id = ? AND status = 'cancelled'`,
+      [requestId]
+    );
+    return this.getById(requestId);
   }
 };