✨ 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/Infrastructure/TakeoutSaaS.Infrastructure/App/Persistence/TakeoutAppDbContext.cs

@@ -24,6 +24,7 @@ using TakeoutSaaS.Infrastructure.Common.Persistence;
 using TakeoutSaaS.Shared.Abstractions.Ids;
 using TakeoutSaaS.Shared.Abstractions.Security;
 using TakeoutSaaS.Shared.Abstractions.Tenancy;
+using TakeoutSaaS.Infrastructure.App.Persistence.Configurations;
 
 namespace TakeoutSaaS.Infrastructure.App.Persistence;
 
@@ -762,28 +763,12 @@ public sealed class TakeoutAppDbContext(
 
     private static void ConfigureTenantBilling(EntityTypeBuilder<TenantBillingStatement> builder)
     {
-        builder.ToTable("tenant_billing_statements");
-        builder.HasKey(x => x.Id);
-        builder.Property(x => x.StatementNo).HasMaxLength(64).IsRequired();
-        builder.Property(x => x.AmountDue).HasPrecision(18, 2);
-        builder.Property(x => x.AmountPaid).HasPrecision(18, 2);
-        builder.Property(x => x.Status).HasConversion<int>();
-        builder.Property(x => x.LineItemsJson).HasColumnType("text");
-        builder.HasIndex(x => new { x.TenantId, x.StatementNo }).IsUnique();
+        new TenantBillingStatementConfiguration().Configure(builder);
     }
 
     private static void ConfigureTenantPayment(EntityTypeBuilder<TenantPayment> builder)
     {
-        builder.ToTable("tenant_payments");
-        builder.HasKey(x => x.Id);
-        builder.Property(x => x.BillingStatementId).IsRequired();
-        builder.Property(x => x.Amount).HasPrecision(18, 2).IsRequired();
-        builder.Property(x => x.Method).HasConversion<int>();
-        builder.Property(x => x.Status).HasConversion<int>();
-        builder.Property(x => x.TransactionNo).HasMaxLength(64);
-        builder.Property(x => x.ProofUrl).HasMaxLength(512);
-        builder.Property(x => x.Notes).HasMaxLength(512);
-        builder.HasIndex(x => new { x.TenantId, x.BillingStatementId });
+        new TenantPaymentConfiguration().Configure(builder);
     }
 
     private static void ConfigureTenantNotification(EntityTypeBuilder<TenantNotification> builder)