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

@@ -0,0 +1,35 @@
+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);
+    }
+}