feat(sprint-004): 월간 비교·확인·정산 백엔드 (Section A) + Mock 해제

Backend:
- monthly_work_confirmations 테이블 마이그레이션
- monthlyComparisonModel: 비교 쿼리 8개 (보고서/근태/확인 병렬 조회)
- monthlyComparisonController: 5 API (my-records/records/confirm/all-status/export)
- 일별 7상태 판정 (match/mismatch/report_only/attend_only/vacation/holiday/none)
- 확인/반려 UPSERT + 반려 시 알림 (단일 트랜잭션)
- 엑셀 2시트 (exceljs) + 헤더 스타일 + 불일치/휴가 행 색상
- support_team+ 권한 체크 (all-status, export)
- exceljs 의존성 추가

Frontend:
- monthly-comparison.js MOCK_ENABLED = false (API 연결)

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

system1-factory/api/db/migrations/20260330_create_monthly_work_confirmations.sql

@@ -0,0 +1,21 @@
+CREATE TABLE IF NOT EXISTS monthly_work_confirmations (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  user_id INT NOT NULL COMMENT '작업자 user_id (workers.user_id)',
+  year INT NOT NULL,
+  month INT NOT NULL,
+  status ENUM('pending', 'confirmed', 'rejected') NOT NULL DEFAULT 'pending',
+  total_work_days INT DEFAULT 0 COMMENT '총 근무일수',
+  total_work_hours DECIMAL(6,2) DEFAULT 0 COMMENT '총 근무시간',
+  total_overtime_hours DECIMAL(6,2) DEFAULT 0 COMMENT '총 연장근로시간',
+  vacation_days DECIMAL(4,2) DEFAULT 0 COMMENT '휴가 일수',
+  mismatch_count INT DEFAULT 0 COMMENT '불일치 건수',
+  reject_reason TEXT NULL COMMENT '반려 사유',
+  confirmed_at TIMESTAMP NULL COMMENT '확인 일시',
+  rejected_at TIMESTAMP NULL COMMENT '반려 일시',
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  UNIQUE KEY uq_user_year_month (user_id, year, month),
+  KEY idx_year_month (year, month),
+  KEY idx_status (status)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci
+COMMENT='월간 근무 확인 (승인/반려)'