fix: 劳务协议人员社保基数为0时仍按基本工资计算的bug

根因：socialInsBase || baseSalary 中 0 是 falsy，导致基数为0
的劳务协议/实习人员回退到基本工资计算社保。

修复：改为 nullish check（!= null），区分"明确设置为0"
和"未设置(null)"：
- socialInsBase = 0 → 用 0（劳务协议不交社保）
- socialInsBase = null → 用 baseSalary（回退）
- socialInsBase = 5000 → 用 5000（核定基数）

涉及文件：
- payroll.service.ts calcBatchEntry + prePayrollCheck
- payroll.routes.ts 旧版薪资计算

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

backend/src/services/payroll.service.ts

@@ -233,9 +233,9 @@ export async function calcBatchEntry(
     })
   }
 
-  // 社保基数：优先用员工核定基数，否则用基本工资
-  const socialBase = employee.socialInsBase || inputs.baseSalary
-  const housingBase = employee.housingFundBase || inputs.baseSalary
+  // 社保基数：优先用员工核定基数（含0，劳务协议人员基数为0不交社保），未设置时回退到基本工资
+  const socialBase = employee.socialInsBase != null ? employee.socialInsBase : inputs.baseSalary
+  const housingBase = employee.housingFundBase != null ? employee.housingFundBase : inputs.baseSalary
 
   let socialEmp = 0, socialOrg = 0, housingEmp = 0, housingOrg = 0
 
@@ -689,7 +689,7 @@ export async function prePayrollCheck(orgId: string, batchId: string): Promise<P
   // 1. 社保基数是否在上下限范围内
   if (socialConfig) {
     for (const entry of entries) {
-      const base = entry.employee.socialInsBase || entry.baseSalary
+      const base = entry.employee.socialInsBase != null ? entry.employee.socialInsBase : entry.baseSalary
       if (base < socialConfig.baseMin || base > socialConfig.baseMax) {
         checks.push({
           code: 'SOCIAL_BASE_OUT_OF_RANGE',
@@ -707,7 +707,7 @@ export async function prePayrollCheck(orgId: string, batchId: string): Promise<P
   // 2. 公积金基数是否在上下限范围内
   if (housingConfig) {
     for (const entry of entries) {
-      const base = entry.employee.housingFundBase || entry.baseSalary
+      const base = entry.employee.housingFundBase != null ? entry.employee.housingFundBase : entry.baseSalary
       if (base < housingConfig.baseMin || base > housingConfig.baseMax) {
         checks.push({
           code: 'HOUSING_BASE_OUT_OF_RANGE',