✨ 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/Configurations/TenantBillingStatementConfiguration.cs

@@ -0,0 +1,51 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using TakeoutSaaS.Domain.Tenants.Entities;
+using TakeoutSaaS.Domain.Tenants.Enums;
+
+namespace TakeoutSaaS.Infrastructure.App.Persistence.Configurations;
+
+/// <summary>
+/// <see cref="TenantBillingStatement"/> EF Core 映射配置。
+/// </summary>
+public sealed class TenantBillingStatementConfiguration : IEntityTypeConfiguration<TenantBillingStatement>
+{
+    /// <inheritdoc />
+    public void Configure(EntityTypeBuilder<TenantBillingStatement> builder)
+    {
+        builder.ToTable("tenant_billing_statements");
+        builder.HasKey(x => x.Id);
+
+        // 1. 字段约束
+        builder.Property(x => x.StatementNo).HasMaxLength(64).IsRequired();
+        builder.Property(x => x.BillingType).HasConversion<int>();
+        builder.Property(x => x.AmountDue).HasPrecision(18, 2);
+        builder.Property(x => x.DiscountAmount).HasPrecision(18, 2);
+        builder.Property(x => x.TaxAmount).HasPrecision(18, 2);
+        builder.Property(x => x.AmountPaid).HasPrecision(18, 2);
+        builder.Property(x => x.Currency).HasMaxLength(8).HasDefaultValue("CNY");
+        builder.Property(x => x.Status).HasConversion<int>();
+
+        // 2. (空行后) JSON 字段（当前以 text 存储 JSON 字符串，便于兼容历史迁移）
+        builder.Property(x => x.LineItemsJson).HasColumnType("text");
+
+        // 3. (空行后) 备注字段
+        builder.Property(x => x.Notes).HasMaxLength(512);
+
+        // 4. (空行后) 唯一约束与索引
+        builder.HasIndex(x => new { x.TenantId, x.StatementNo }).IsUnique();
+
+        // 5. (空行后) 性能索引（高频查询：租户+状态+到期日）
+        builder.HasIndex(x => new { x.TenantId, x.Status, x.DueDate })
+            .HasDatabaseName("idx_billing_tenant_status_duedate");
+
+        // 6. (空行后) 逾期扫描索引（仅索引 Pending/Overdue）
+        builder.HasIndex(x => new { x.Status, x.DueDate })
+            .HasDatabaseName("idx_billing_status_duedate")
+            .HasFilter($"\"Status\" IN ({(int)TenantBillingStatus.Pending}, {(int)TenantBillingStatus.Overdue})");
+
+        // 7. (空行后) 创建时间索引（支持列表倒序）
+        builder.HasIndex(x => x.CreatedAt)
+            .HasDatabaseName("idx_billing_created_at");
+    }
+}

src/Infrastructure/TakeoutSaaS.Infrastructure/App/Persistence/Configurations/TenantPaymentConfiguration.cs

@@ -0,0 +1,40 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using TakeoutSaaS.Domain.Tenants.Entities;
+
+namespace TakeoutSaaS.Infrastructure.App.Persistence.Configurations;
+
+/// <summary>
+/// <see cref="TenantPayment"/> EF Core 映射配置。
+/// </summary>
+public sealed class TenantPaymentConfiguration : IEntityTypeConfiguration<TenantPayment>
+{
+    /// <inheritdoc />
+    public void Configure(EntityTypeBuilder<TenantPayment> builder)
+    {
+        builder.ToTable("tenant_payments");
+        builder.HasKey(x => x.Id);
+
+        // 1. 字段约束
+        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.RefundReason).HasMaxLength(512);
+        builder.Property(x => x.Notes).HasMaxLength(512);
+
+        // 2. (空行后) 复合索引：租户+账单
+        builder.HasIndex(x => new { x.TenantId, x.BillingStatementId });
+
+        // 3. (空行后) 支付记录时间排序索引
+        builder.HasIndex(x => new { x.BillingStatementId, x.PaidAt })
+            .HasDatabaseName("idx_payment_billing_paidat");
+
+        // 4. (空行后) 交易号索引（部分索引：仅非空）
+        builder.HasIndex(x => x.TransactionNo)
+            .HasDatabaseName("idx_payment_transaction_no")
+            .HasFilter("\"TransactionNo\" IS NOT NULL");
+    }
+}