fix: 个税累计预扣中专项附加扣除改为按月读取实际填报金额

原逻辑用 employee.specialDeduction（便捷字段）× 月数计算累计
专项附加扣除，未按月读取 SpecialDeductionRecord 实际金额。
若员工某月填报/取消专项附加扣除，累计值会不准。

修复：
- 优先按月查询 SpecialDeductionRecord（当年至当月）累加实际金额
- 无按月记录时回退到便捷字段 × 月数（兼容旧数据）
- taxBreakdown 增加 specialDeductionSource 字段标识数据来源

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

backend/src/services/payroll.service.ts

@@ -258,7 +258,22 @@ export async function calcBatchEntry(
     const ytdIncome = archivedEntries.reduce((s, e) => s + e.totalPay, 0) + totalPay
     const ytdSocialEmp = archivedEntries.reduce((s, e) => s + e.socialEmp, 0) + socialEmp
     const ytdHousingEmp = archivedEntries.reduce((s, e) => s + e.housingEmp, 0) + housingEmp
-    const ytdSpecialDeduction = employee.specialDeduction * Number(month.slice(5, 7))
+
+    // 专项附加扣除：按月读取 SpecialDeductionRecord 实际填报金额累加
+    // 优先使用按月记录；若无按月记录，回退到员工便捷字段 × 月数（兼容旧数据）
+    const deductionRecords = await prisma.specialDeductionRecord.findMany({
+      where: { orgId, employeeId, month: { startsWith: year, lte: month } },
+      select: { amount: true, month: true },
+    })
+    let ytdSpecialDeduction: number
+    if (deductionRecords.length > 0) {
+      // 按月实际填报金额累加
+      ytdSpecialDeduction = deductionRecords.reduce((s, r) => s + r.amount, 0)
+    } else {
+      // 回退：员工便捷字段 × 月数（兼容未填报按月记录的旧数据）
+      ytdSpecialDeduction = employee.specialDeduction * Number(month.slice(5, 7))
+    }
+
     const ytdTaxDeducted = archivedEntries.reduce((s, e) => s + e.tax, 0)
     const deductionAmount = 5000 * Number(month.slice(5, 7))
     const ytdTaxableIncome = Math.max(0, ytdIncome - deductionAmount - ytdSocialEmp - ytdHousingEmp - ytdSpecialDeduction)
@@ -271,6 +286,8 @@ export async function calcBatchEntry(
       ytdSocialEmp,
       ytdHousingEmp,
       ytdSpecialDeduction,
+      specialDeductionSource: deductionRecords.length > 0 ? '按月记录' : '便捷字段',
+      specialDeductionRecords: deductionRecords.length,
       ytdTaxableIncome,
       ytdTaxDeducted,
       currentMonthTax: tax,