✨ 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/CreateBillingCommandHandler.cs

@@ -0,0 +1,65 @@
+using MediatR;
+using System.Text.Json;
+using TakeoutSaaS.Application.App.Billings.Commands;
+using TakeoutSaaS.Application.App.Billings.Dto;
+using TakeoutSaaS.Domain.Tenants.Entities;
+using TakeoutSaaS.Domain.Tenants.Enums;
+using TakeoutSaaS.Domain.Tenants.Repositories;
+using TakeoutSaaS.Shared.Abstractions.Constants;
+using TakeoutSaaS.Shared.Abstractions.Exceptions;
+using TakeoutSaaS.Shared.Abstractions.Ids;
+
+namespace TakeoutSaaS.Application.App.Billings.Handlers;
+
+/// <summary>
+/// 创建账单命令处理器。
+/// </summary>
+public sealed class CreateBillingCommandHandler(
+    ITenantRepository tenantRepository,
+    ITenantBillingRepository billingRepository,
+    IIdGenerator idGenerator)
+    : IRequestHandler<CreateBillingCommand, BillingDetailDto>
+{
+    /// <inheritdoc />
+    public async Task<BillingDetailDto> Handle(CreateBillingCommand request, CancellationToken cancellationToken)
+    {
+        // 1. 校验租户存在
+        var tenant = await tenantRepository.FindByIdAsync(request.TenantId, cancellationToken);
+        if (tenant is null)
+        {
+            throw new BusinessException(ErrorCodes.NotFound, "租户不存在");
+        }
+
+        // 2. (空行后) 构建账单实体
+        var now = DateTime.UtcNow;
+        var statementNo = $"BIL-{now:yyyyMMdd}-{idGenerator.NextId()}";
+        var lineItemsJson = JsonSerializer.Serialize(request.LineItems);
+
+        var billing = new TenantBillingStatement
+        {
+            TenantId = request.TenantId,
+            StatementNo = statementNo,
+            BillingType = request.BillingType,
+            SubscriptionId = null,
+            PeriodStart = now,
+            PeriodEnd = now,
+            AmountDue = request.AmountDue,
+            DiscountAmount = 0m,
+            TaxAmount = 0m,
+            AmountPaid = 0m,
+            Currency = "CNY",
+            Status = TenantBillingStatus.Pending,
+            DueDate = request.DueDate,
+            LineItemsJson = lineItemsJson,
+            Notes = request.Notes
+        };
+
+        // 3. (空行后) 持久化账单
+        await billingRepository.AddAsync(billing, cancellationToken);
+        await billingRepository.SaveChangesAsync(cancellationToken);
+
+        // 4. (空行后) 返回详情 DTO
+        return billing.ToBillingDetailDto([], tenant.Name);
+    }
+}
+