重构 API 为 8 领域 + 应用层架构

将后端模块拆分为 domains、applications、shared 三层，结算计算器移入 domain 纯函数目录，API 路径与测试保持不变。

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/api/src/integration.spec.ts

@@ -2,7 +2,7 @@ import {
   settleSelection,
   calculatePayout,
   isQuarterHandicapOrTotal,
-} from './settlement/settlement-calculator';
+} from './domains/settlement/domain/settlement-calculator';
 
 /**
  * Agent credit & wallet integration scenarios (A001-A007)