feat: finalize core modules and gateway

src/Application/TakeoutSaaS.Application/Messaging/Abstractions/IEventPublisher.cs

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+using System.Threading;
+using System.Threading.Tasks;
+
+namespace TakeoutSaaS.Application.Messaging.Abstractions;
+
+/// <summary>
+/// 领域事件发布抽象。
+/// </summary>
+public interface IEventPublisher
+{
+    /// <summary>
+    /// 发布领域事件。
+    /// </summary>
+    Task PublishAsync<TEvent>(string routingKey, TEvent @event, CancellationToken cancellationToken = default);
+}

src/Application/TakeoutSaaS.Application/Messaging/EventRoutingKeys.cs

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+namespace TakeoutSaaS.Application.Messaging;
+
+/// <summary>
+/// 事件路由键常量。
+/// </summary>
+public static class EventRoutingKeys
+{
+    /// <summary>
+    /// 订单创建事件路由键。
+    /// </summary>
+    public const string OrderCreated = "orders.created";
+
+    /// <summary>
+    /// 支付成功事件路由键。
+    /// </summary>
+    public const string PaymentSucceeded = "payments.succeeded";
+}

src/Application/TakeoutSaaS.Application/Messaging/Events/OrderCreatedEvent.cs

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+namespace TakeoutSaaS.Application.Messaging.Events;
+
+/// <summary>
+/// 订单创建事件。
+/// </summary>
+public sealed class OrderCreatedEvent
+{
+    /// <summary>
+    /// 订单标识。
+    /// </summary>
+    public Guid OrderId { get; init; }
+
+    /// <summary>
+    /// 订单编号。
+    /// </summary>
+    public string OrderNo { get; init; } = string.Empty;
+
+    /// <summary>
+    /// 实付金额。
+    /// </summary>
+    public decimal Amount { get; init; }
+
+    /// <summary>
+    /// 所属租户。
+    /// </summary>
+    public Guid TenantId { get; init; }
+
+    /// <summary>
+    /// 创建时间（UTC）。
+    /// </summary>
+    public DateTime CreatedAt { get; init; }
+}

src/Application/TakeoutSaaS.Application/Messaging/Events/PaymentSucceededEvent.cs

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+namespace TakeoutSaaS.Application.Messaging.Events;
+
+/// <summary>
+/// 支付成功事件。
+/// </summary>
+public sealed class PaymentSucceededEvent
+{
+    /// <summary>
+    /// 订单标识。
+    /// </summary>
+    public Guid OrderId { get; init; }
+
+    /// <summary>
+    /// 支付流水号。
+    /// </summary>
+    public string PaymentNo { get; init; } = string.Empty;
+
+    /// <summary>
+    /// 支付金额。
+    /// </summary>
+    public decimal Amount { get; init; }
+
+    /// <summary>
+    /// 所属租户。
+    /// </summary>
+    public Guid TenantId { get; init; }
+
+    /// <summary>
+    /// 支付时间（UTC）。
+    /// </summary>
+    public DateTime PaidAt { get; init; }
+}

src/Application/TakeoutSaaS.Application/Messaging/Extensions/MessagingServiceCollectionExtensions.cs

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+using Microsoft.Extensions.DependencyInjection;
+using TakeoutSaaS.Application.Messaging.Abstractions;
+using TakeoutSaaS.Application.Messaging.Services;
+
+namespace TakeoutSaaS.Application.Messaging.Extensions;
+
+/// <summary>
+/// 消息模块应用层注册。
+/// </summary>
+public static class MessagingServiceCollectionExtensions
+{
+    /// <summary>
+    /// 注册事件发布器。
+    /// </summary>
+    public static IServiceCollection AddMessagingApplication(this IServiceCollection services)
+    {
+        services.AddScoped<IEventPublisher, EventPublisher>();
+        return services;
+    }
+}

src/Application/TakeoutSaaS.Application/Messaging/Services/EventPublisher.cs

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+using System.Threading;
+using System.Threading.Tasks;
+using TakeoutSaaS.Application.Messaging.Abstractions;
+using TakeoutSaaS.Module.Messaging.Abstractions;
+
+namespace TakeoutSaaS.Application.Messaging.Services;
+
+/// <summary>
+/// 事件发布适配器，封装应用层到 MQ 的发布。
+/// </summary>
+public sealed class EventPublisher(IMessagePublisher messagePublisher) : IEventPublisher
+{
+    /// <inheritdoc />
+    public Task PublishAsync<TEvent>(string routingKey, TEvent @event, CancellationToken cancellationToken = default)
+        => messagePublisher.PublishAsync(routingKey, @event, cancellationToken);
+}