feat: 新增租户端商户中心聚合接口

提供 /merchant/info 聚合查询，返回商户主体、资质、合同、员工与日志信息，满足租户端商户中心页面一次加载全部相关数据。

src/Api/TakeoutSaaS.TenantApi/Controllers/MerchantController.cs

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+using MediatR;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TakeoutSaaS.Application.App.Merchants.Dto;
+using TakeoutSaaS.Application.App.Merchants.Queries;
+using TakeoutSaaS.Shared.Abstractions.Results;
+using TakeoutSaaS.Shared.Web.Api;
+
+namespace TakeoutSaaS.TenantApi.Controllers;
+
+/// <summary>
+/// 租户端商户中心。
+/// </summary>
+[ApiVersion("1.0")]
+[Authorize]
+[Route("api/tenant/v{version:apiVersion}/merchant")]
+public sealed class MerchantController(IMediator mediator) : BaseApiController
+{
+    /// <summary>
+    /// 获取当前登录用户对应的商户中心信息。
+    /// </summary>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>商户中心聚合信息。</returns>
+    [HttpGet("info")]
+    [ProducesResponseType(typeof(ApiResponse<CurrentMerchantCenterDto>), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ApiResponse<CurrentMerchantCenterDto>), StatusCodes.Status401Unauthorized)]
+    public async Task<ApiResponse<CurrentMerchantCenterDto>> GetInfo(CancellationToken cancellationToken)
+    {
+        // 1. 查询当前商户中心信息
+        var info = await mediator.Send(new GetCurrentMerchantCenterQuery(), cancellationToken);
+
+        // 2. 返回聚合信息
+        return ApiResponse<CurrentMerchantCenterDto>.Ok(info);
+    }
+}

src/Application/TakeoutSaaS.Application/App/Merchants/Dto/CurrentMerchantCenterDto.cs

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+using TakeoutSaaS.Application.App.Stores.Dto;
+
+namespace TakeoutSaaS.Application.App.Merchants.Dto;
+
+/// <summary>
+/// 当前租户商户中心聚合信息。
+/// </summary>
+public sealed class CurrentMerchantCenterDto
+{
+    /// <summary>
+    /// 商户主体详情。
+    /// </summary>
+    public MerchantDetailDto Merchant { get; init; } = new();
+
+    /// <summary>
+    /// 商户证照列表。
+    /// </summary>
+    public IReadOnlyList<MerchantDocumentDto> Documents { get; init; } = [];
+
+    /// <summary>
+    /// 商户合同列表。
+    /// </summary>
+    public IReadOnlyList<MerchantContractDto> Contracts { get; init; } = [];
+
+    /// <summary>
+    /// 商户员工列表。
+    /// </summary>
+    public IReadOnlyList<StoreStaffDto> Staffs { get; init; } = [];
+
+    /// <summary>
+    /// 商户审核日志列表。
+    /// </summary>
+    public IReadOnlyList<MerchantAuditLogDto> AuditLogs { get; init; } = [];
+
+    /// <summary>
+    /// 商户变更日志列表。
+    /// </summary>
+    public IReadOnlyList<MerchantChangeLogDto> ChangeLogs { get; init; } = [];
+}

src/Application/TakeoutSaaS.Application/App/Merchants/Handlers/GetCurrentMerchantCenterQueryHandler.cs

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+using MediatR;
+using TakeoutSaaS.Application.App.Merchants.Dto;
+using TakeoutSaaS.Application.App.Merchants.Queries;
+using TakeoutSaaS.Application.App.Stores;
+using TakeoutSaaS.Application.App.Stores.Dto;
+using TakeoutSaaS.Domain.Identity.Repositories;
+using TakeoutSaaS.Domain.Merchants.Repositories;
+using TakeoutSaaS.Domain.Stores.Repositories;
+using TakeoutSaaS.Domain.Tenants.Repositories;
+using TakeoutSaaS.Shared.Abstractions.Constants;
+using TakeoutSaaS.Shared.Abstractions.Exceptions;
+using TakeoutSaaS.Shared.Abstractions.Security;
+using TakeoutSaaS.Shared.Abstractions.Tenancy;
+
+namespace TakeoutSaaS.Application.App.Merchants.Handlers;
+
+/// <summary>
+/// 获取当前商户中心信息查询处理器。
+/// </summary>
+public sealed class GetCurrentMerchantCenterQueryHandler(
+    ICurrentUserAccessor currentUserAccessor,
+    ITenantProvider tenantProvider,
+    IIdentityUserRepository identityUserRepository,
+    IMerchantRepository merchantRepository,
+    IStoreRepository storeRepository,
+    ITenantRepository tenantRepository)
+    : IRequestHandler<GetCurrentMerchantCenterQuery, CurrentMerchantCenterDto>
+{
+    /// <summary>
+    /// 获取当前登录用户可访问的商户中心完整信息。
+    /// </summary>
+    /// <param name="request">查询请求。</param>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>商户中心聚合信息。</returns>
+    public async Task<CurrentMerchantCenterDto> Handle(GetCurrentMerchantCenterQuery request, CancellationToken cancellationToken)
+    {
+        // 1. 校验登录上下文
+        var currentUserId = currentUserAccessor.UserId;
+        if (currentUserId <= 0)
+        {
+            throw new BusinessException(ErrorCodes.Unauthorized, "未登录或登录已过期");
+        }
+
+        // 2. 校验租户上下文
+        var currentTenantId = tenantProvider.GetCurrentTenantId();
+        if (currentTenantId <= 0)
+        {
+            throw new BusinessException(ErrorCodes.BadRequest, "缺少租户标识，请在 Header 指定租户");
+        }
+
+        // 3. 校验当前用户归属
+        var currentUser = await identityUserRepository.FindByIdAsync(currentUserId, cancellationToken)
+            ?? throw new BusinessException(ErrorCodes.NotFound, "用户不存在");
+        if (currentUser.TenantId != currentTenantId)
+        {
+            throw new BusinessException(ErrorCodes.Unauthorized, "无权访问当前租户的商户信息");
+        }
+
+        // 4. 解析当前用户可访问商户
+        var merchantId = await ResolveMerchantIdAsync(currentUser.MerchantId, currentTenantId, cancellationToken);
+
+        // 5. 读取商户主体与租户信息
+        var merchant = await merchantRepository.FindByIdAsync(merchantId, currentTenantId, cancellationToken)
+            ?? throw new BusinessException(ErrorCodes.NotFound, "商户不存在");
+        var tenant = await tenantRepository.FindByIdAsync(currentTenantId, cancellationToken);
+
+        // 6. 读取商户关联数据
+        var stores = await storeRepository.GetByMerchantIdAsync(merchantId, currentTenantId, cancellationToken);
+        var staffs = await merchantRepository.GetStaffAsync(merchantId, currentTenantId, cancellationToken);
+        var documents = await merchantRepository.GetDocumentsAsync(merchantId, currentTenantId, cancellationToken);
+        var contracts = await merchantRepository.GetContractsAsync(merchantId, currentTenantId, cancellationToken);
+        var auditLogs = await merchantRepository.GetAuditLogsAsync(merchantId, currentTenantId, cancellationToken);
+        var changeLogs = await merchantRepository.GetChangeLogsAsync(merchantId, currentTenantId, null, cancellationToken);
+
+        // 7. 组装并返回聚合结果
+        var storeDtos = MerchantMapping.ToStoreDtos(stores);
+        var staffDtos = staffs.Select(StoreMapping.ToDto).ToList();
+        return new CurrentMerchantCenterDto
+        {
+            Merchant = MerchantMapping.ToDetailDto(merchant, tenant?.Name, storeDtos),
+            Staffs = staffDtos,
+            Documents = MerchantMapping.ToDocumentDtos(documents),
+            Contracts = MerchantMapping.ToContractDtos(contracts),
+            AuditLogs = auditLogs.Select(MerchantMapping.ToDto).ToList(),
+            ChangeLogs = changeLogs.Select(MerchantMapping.ToDto).ToList()
+        };
+    }
+
+    /// <summary>
+    /// 解析当前用户对应的商户标识。
+    /// </summary>
+    /// <param name="currentUserMerchantId">当前用户绑定的商户 ID。</param>
+    /// <param name="tenantId">当前租户 ID。</param>
+    /// <param name="cancellationToken">取消标记。</param>
+    /// <returns>商户 ID。</returns>
+    private async Task<long> ResolveMerchantIdAsync(long? currentUserMerchantId, long tenantId, CancellationToken cancellationToken)
+    {
+        // 1. 优先使用用户显式绑定的商户
+        if (currentUserMerchantId is > 0)
+        {
+            return currentUserMerchantId.Value;
+        }
+
+        // 2. 兜底读取租户下最新商户
+        var merchants = await merchantRepository.SearchAsync(tenantId, status: null, cancellationToken);
+        var merchantId = merchants.FirstOrDefault()?.Id;
+        if (!merchantId.HasValue || merchantId.Value <= 0)
+        {
+            throw new BusinessException(ErrorCodes.NotFound, "当前租户尚未创建商户信息");
+        }
+
+        // 3. 返回商户标识
+        return merchantId.Value;
+    }
+}

src/Application/TakeoutSaaS.Application/App/Merchants/Queries/GetCurrentMerchantCenterQuery.cs

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+using MediatR;
+using TakeoutSaaS.Application.App.Merchants.Dto;
+
+namespace TakeoutSaaS.Application.App.Merchants.Queries;
+
+/// <summary>
+/// 获取当前登录用户可访问的商户中心聚合信息。
+/// </summary>
+public sealed record GetCurrentMerchantCenterQuery : IRequest<CurrentMerchantCenterDto>;