perf(finance): 统一把 Intl.DateTimeFormat 提到模块级，消除逐行构造开销

承接 14b595b0。以下 7 处仍在函数体内 `new Intl.DateTimeFormat(...)`，
单次构造约 0.5ms，按行/按条调用时会线性放大：

- utils/finance-date-range.ts         formatShanghaiDate
- utils/http.ts                       （已在上一个提交修复）
- scheduler/cost-finance-sync-job.ts  today
- finance/sales-facts-service.ts      formatDateOnly
- finance/order-refund-service.ts     toChinaDate
- admin/admin-finance-service.ts      chinaDateParts
- admin/admin-dashboard-service.ts    resolveChinaTodayRange
- scripts/sync-cost-finance.ts        formatShanghaiDate

均为同选项的纯 hoist，输出逐字节不变：formatShanghaiDate 用 9 例跨时区/
毫秒/闰日语料与旧实现比对一致；其余为相同 en-CA/en-US + Asia/Shanghai
选项的等价替换，formatToParts 的取值改为单次遍历、去掉 fromEntries/reduce。

apps/backend/scripts/sync-cost-finance.ts

@@ -78,11 +78,14 @@ function normalizeDays(value: unknown) {
   return parsed
 }
 
+/** 模块级只构造一次：`new Intl.DateTimeFormat()` 每次约 0.5ms，逐条调用会线性放大。 */
+const SHANGHAI_DATE_FORMATTER = new Intl.DateTimeFormat('en-CA', {
+  timeZone: 'Asia/Shanghai',
+  year: 'numeric',
+  month: '2-digit',
+  day: '2-digit',
+})
+
 function formatShanghaiDate(value: Date) {
-  return new Intl.DateTimeFormat('en-CA', {
-    timeZone: 'Asia/Shanghai',
-    year: 'numeric',
-    month: '2-digit',
-    day: '2-digit',
-  }).format(value)
+  return SHANGHAI_DATE_FORMATTER.format(value)
 }