feat(sprint005): 월간 확인 워크플로우 — 관리자 확인요청 + 수정요청

- DB: status ENUM 확장 (review_sent, change_request) + reviewed_by/at, change_details
- API: POST /review-send (일괄 확인요청), POST /review-respond (수정 승인/거부)
- 작업자: pending=검토대기, review_sent=확인/수정요청, rejected=동의(재확인)
- 관리자: 필터 탭 확장 + 확인요청 일괄 발송 버튼
- confirm 상태 전환 검증: pending→confirmed 차단

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

system1-factory/api/db/migrations/20260401_monthly_confirm_workflow.sql

@@ -0,0 +1,6 @@
+-- 월간 확인 워크플로우 확장: pending → review_sent → confirmed/change_request/rejected
+ALTER TABLE monthly_work_confirmations
+  MODIFY status ENUM('pending','review_sent','confirmed','change_request','rejected') DEFAULT 'pending';
+ALTER TABLE monthly_work_confirmations ADD COLUMN IF NOT EXISTS reviewed_by INT NULL;
+ALTER TABLE monthly_work_confirmations ADD COLUMN IF NOT EXISTS reviewed_at TIMESTAMP NULL;
+ALTER TABLE monthly_work_confirmations ADD COLUMN IF NOT EXISTS change_details TEXT NULL;