✨ 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/GetBillingPaymentsQueryHandler.cs

@@ -0,0 +1,139 @@
+using MediatR;
+using System.Data;
+using System.Data.Common;
+using TakeoutSaaS.Application.App.Billings.Dto;
+using TakeoutSaaS.Application.App.Billings.Queries;
+using TakeoutSaaS.Shared.Abstractions.Constants;
+using TakeoutSaaS.Shared.Abstractions.Data;
+using TakeoutSaaS.Shared.Abstractions.Exceptions;
+using TakeoutSaaS.Domain.Tenants.Enums;
+
+namespace TakeoutSaaS.Application.App.Billings.Handlers;
+
+/// <summary>
+/// 查询账单支付记录处理器。
+/// </summary>
+public sealed class GetBillingPaymentsQueryHandler(
+    IDapperExecutor dapperExecutor)
+    : IRequestHandler<GetBillingPaymentsQuery, List<PaymentRecordDto>>
+{
+    /// <summary>
+    /// 处理查询账单支付记录请求。
+    /// </summary>
+    /// <param name="request">查询命令。</param>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>支付记录列表 DTO。</returns>
+    public async Task<List<PaymentRecordDto>> Handle(GetBillingPaymentsQuery request, CancellationToken cancellationToken)
+    {
+        // 1. 校验账单是否存在
+        return await dapperExecutor.QueryAsync(
+            DatabaseConstants.AppDataSource,
+            DatabaseConnectionRole.Read,
+            async (connection, token) =>
+            {
+                // 1.1 校验账单存在
+                var exists = await ExecuteScalarIntAsync(
+                    connection,
+                    """
+                    select 1
+                    from public.tenant_billing_statements b
+                    where b."DeletedAt" is null
+                      and b."Id" = @billingId
+                    limit 1;
+                    """,
+                    [
+                        ("billingId", request.BillingId)
+                    ],
+                    token);
+
+                if (exists == 0)
+                {
+                    throw new BusinessException(ErrorCodes.NotFound, "账单不存在");
+                }
+
+                // 1.2 (空行后) 查询支付记录
+                await using var command = CreateCommand(
+                    connection,
+                    """
+                    select
+                      p."Id",
+                      p."TenantId",
+                      p."BillingStatementId",
+                      p."Amount",
+                      p."Method",
+                      p."Status",
+                      p."TransactionNo",
+                      p."ProofUrl",
+                      p."Notes",
+                      p."VerifiedBy",
+                      p."VerifiedAt",
+                      p."RefundReason",
+                      p."RefundedAt",
+                      p."PaidAt",
+                      p."CreatedAt"
+                    from public.tenant_payments p
+                    where p."DeletedAt" is null
+                      and p."BillingStatementId" = @billingId
+                    order by p."CreatedAt" desc;
+                    """,
+                    [
+                        ("billingId", request.BillingId)
+                    ]);
+
+                await using var reader = await command.ExecuteReaderAsync(token);
+                var results = new List<PaymentRecordDto>();
+                while (await reader.ReadAsync(token))
+                {
+                    results.Add(new PaymentRecordDto
+                    {
+                        Id = reader.GetInt64(0),
+                        TenantId = reader.GetInt64(1),
+                        BillingId = reader.GetInt64(2),
+                        Amount = reader.GetDecimal(3),
+                        Method = (TenantPaymentMethod)reader.GetInt32(4),
+                        Status = (TenantPaymentStatus)reader.GetInt32(5),
+                        TransactionNo = reader.IsDBNull(6) ? null : reader.GetString(6),
+                        ProofUrl = reader.IsDBNull(7) ? null : reader.GetString(7),
+                        Notes = reader.IsDBNull(8) ? null : reader.GetString(8),
+                        VerifiedBy = reader.IsDBNull(9) ? null : reader.GetInt64(9),
+                        VerifiedAt = reader.IsDBNull(10) ? null : reader.GetDateTime(10),
+                        RefundReason = reader.IsDBNull(11) ? null : reader.GetString(11),
+                        RefundedAt = reader.IsDBNull(12) ? null : reader.GetDateTime(12),
+                        PaidAt = reader.IsDBNull(13) ? null : reader.GetDateTime(13),
+                        IsVerified = !reader.IsDBNull(10),
+                        CreatedAt = reader.GetDateTime(14)
+                    });
+                }
+
+                return results;
+            },
+            cancellationToken);
+    }
+
+    private static async Task<int> ExecuteScalarIntAsync(
+        IDbConnection connection,
+        string sql,
+        (string Name, object? Value)[] parameters,
+        CancellationToken cancellationToken)
+    {
+        await using var command = CreateCommand(connection, sql, parameters);
+        var result = await command.ExecuteScalarAsync(cancellationToken);
+        return result is null or DBNull ? 0 : Convert.ToInt32(result);
+    }
+
+    private static DbCommand CreateCommand(IDbConnection connection, string sql, (string Name, object? Value)[] parameters)
+    {
+        var command = connection.CreateCommand();
+        command.CommandText = sql;
+
+        foreach (var (name, value) in parameters)
+        {
+            var p = command.CreateParameter();
+            p.ParameterName = name;
+            p.Value = value ?? DBNull.Value;
+            command.Parameters.Add(p);
+        }
+
+        return (DbCommand)command;
+    }
+}