fix(settlement): 操作记录分页、流水动作与坏账幂等加固

- 新增 settlement-operations 合并列表 API，收付/调账接口改为标准分页
- actionable_only 流水在内存过滤后正确分页
- 已结账单流水不再展示补差/冲正快捷动作
- 坏账核销支持 idempotency_key 并写入 result_bill_id
- 补充操作记录、流水与坏账幂等相关测试

app/Http/Requests/Admin/AdminSettlementBillBadDebtRequest.php

@@ -15,6 +15,7 @@ final class AdminSettlementBillBadDebtRequest extends ApiFormRequest
     {
         return [
             'reason' => ['sometimes', 'nullable', 'string', 'max:255'],
+            'idempotency_key' => ['sometimes', 'nullable', 'string', 'max:64'],
         ];
     }
 }