✨ feat: 完成账单管理模块后端功能开发及API优化

核心功能:
- 账单CRUD操作（创建、查询、详情、更新状态、删除）
- 支付记录管理（创建支付、审核支付）
- 批量操作支持（批量更新账单状态）
- 统计分析功能（账单统计、逾期账单查询）
- 导出功能（Excel/PDF/CSV）

API端点 (16个):
- GET /api/admin/v1/billings - 账单列表（分页、筛选、排序）
- POST /api/admin/v1/billings - 创建账单
- GET /api/admin/v1/billings/{id} - 账单详情
- DELETE /api/admin/v1/billings/{id} - 删除账单
- PUT /api/admin/v1/billings/{id}/status - 更新状态
- POST /api/admin/v1/billings/batch/status - 批量更新
- GET /api/admin/v1/billings/{id}/payments - 支付记录
- POST /api/admin/v1/billings/{id}/payments - 创建支付
- PUT /api/admin/v1/billings/payments/{paymentId}/verify - 审核支付
- GET /api/admin/v1/billings/statistics - 统计数据
- GET /api/admin/v1/billings/overdue - 逾期账单
- POST /api/admin/v1/billings/export - 导出账单

架构优化:
- 采用CQRS模式分离读写（MediatR + Dapper + EF Core）
- 完整的领域模型设计（TenantBillingStatement, TenantPayment等）
- FluentValidation请求验证
- 状态机管理账单和支付状态流转

API设计优化 (三项改进):
1. 导出API响应Content-Type改为application/octet-stream
2. 支付审核API添加Approved和Notes可选参数，支持通过/拒绝
3. 移除TenantBillings API中重复的TenantId参数

数据库变更:
- 新增账单相关表及关系
- 支持Snowflake ID主键
- 完整的审计字段支持

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/Application/TakeoutSaaS.Application/App/Billings/Handlers/VerifyPaymentCommandHandler.cs

@@ -0,0 +1,73 @@
+using MediatR;
+using TakeoutSaaS.Application.App.Billings.Commands;
+using TakeoutSaaS.Application.App.Billings.Dto;
+using TakeoutSaaS.Domain.Tenants.Repositories;
+using TakeoutSaaS.Shared.Abstractions.Constants;
+using TakeoutSaaS.Shared.Abstractions.Exceptions;
+using TakeoutSaaS.Shared.Abstractions.Security;
+
+namespace TakeoutSaaS.Application.App.Billings.Handlers;
+
+/// <summary>
+/// 审核支付命令处理器。
+/// </summary>
+public sealed class VerifyPaymentCommandHandler(
+    ITenantPaymentRepository paymentRepository,
+    ITenantBillingRepository billingRepository,
+    ICurrentUserAccessor currentUserAccessor)
+    : IRequestHandler<VerifyPaymentCommand, PaymentRecordDto>
+{
+    /// <inheritdoc />
+    public async Task<PaymentRecordDto> Handle(VerifyPaymentCommand request, CancellationToken cancellationToken)
+    {
+        // 1. 校验操作者身份
+        if (!currentUserAccessor.IsAuthenticated || currentUserAccessor.UserId <= 0)
+        {
+            throw new BusinessException(ErrorCodes.Unauthorized, "未登录或无效的操作者身份");
+        }
+
+        // 2. (空行后) 查询支付记录
+        var payment = await paymentRepository.FindByIdAsync(request.PaymentId, cancellationToken);
+        if (payment is null)
+        {
+            throw new BusinessException(ErrorCodes.NotFound, "支付记录不存在");
+        }
+
+        // 3. (空行后) 查询关联账单
+        var billing = await billingRepository.FindByIdAsync(payment.BillingStatementId, cancellationToken);
+        if (billing is null)
+        {
+            throw new BusinessException(ErrorCodes.NotFound, "关联账单不存在");
+        }
+
+        // 4. (空行后) 归一化审核备注
+        var normalizedNotes = string.IsNullOrWhiteSpace(request.Notes) ? null : request.Notes.Trim();
+
+        // 5. (空行后) 根据审核结果更新支付与账单状态
+        if (request.Approved)
+        {
+            payment.Verify(currentUserAccessor.UserId);
+            payment.Notes = normalizedNotes;
+
+            billing.MarkAsPaid(payment.Amount, payment.TransactionNo ?? string.Empty);
+        }
+        else
+        {
+            payment.Reject(currentUserAccessor.UserId, normalizedNotes ?? string.Empty);
+            payment.Notes = normalizedNotes;
+        }
+
+        // 6. (空行后) 持久化更新状态
+        await paymentRepository.UpdateAsync(payment, cancellationToken);
+        if (request.Approved)
+        {
+            await billingRepository.UpdateAsync(billing, cancellationToken);
+        }
+
+        // 7. (空行后) 保存数据库更改
+        await paymentRepository.SaveChangesAsync(cancellationToken);
+
+        // 8. (空行后) 返回 DTO
+        return payment.ToPaymentRecordDto();
+    }
+}