chore: 初始化项目基础文件

src/Core/TakeoutSaaS.Shared.Abstractions/Constants/ErrorCodes.cs

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+namespace TakeoutSaaS.Shared.Abstractions.Constants;
+
+/// <summary>
+/// 统一错误码常量。
+/// </summary>
+public static class ErrorCodes
+{
+    public const int BadRequest = 400;
+    public const int Unauthorized = 401;
+    public const int Forbidden = 403;
+    public const int NotFound = 404;
+    public const int Conflict = 409;
+    public const int ValidationFailed = 422;
+    public const int InternalServerError = 500;
+
+    // 业务自定义区间（10000+）
+    public const int BusinessError = 10001;
+}
+

src/Core/TakeoutSaaS.Shared.Abstractions/Entities/IAuditableEntity.cs

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+namespace TakeoutSaaS.Shared.Abstractions.Entities;
+
+/// <summary>
+/// 审计字段接口
+/// </summary>
+public interface IAuditableEntity
+{
+    DateTime CreatedAt { get; set; }
+    DateTime? UpdatedAt { get; set; }
+}
+

src/Core/TakeoutSaaS.Shared.Abstractions/Exceptions/BusinessException.cs

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+using System;
+
+namespace TakeoutSaaS.Shared.Abstractions.Exceptions;
+
+/// <summary>
+/// 业务异常（用于可预期的业务校验错误）。
+/// </summary>
+public class BusinessException : Exception
+{
+    /// <summary>
+    /// 业务错误码。
+    /// </summary>
+    public int ErrorCode { get; }
+
+    public BusinessException(int errorCode, string message) : base(message)
+    {
+        ErrorCode = errorCode;
+    }
+}
+

src/Core/TakeoutSaaS.Shared.Abstractions/Exceptions/ValidationException.cs

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+using System;
+using System.Collections.Generic;
+
+namespace TakeoutSaaS.Shared.Abstractions.Exceptions;
+
+/// <summary>
+/// 验证异常（用于聚合验证错误信息）。
+/// </summary>
+public class ValidationException : Exception
+{
+    /// <summary>
+    /// 字段/属性的错误集合。
+    /// </summary>
+    public IDictionary<string, string[]> Errors { get; }
+
+    public ValidationException(IDictionary<string, string[]> errors)
+        : base("一个或多个验证错误")
+    {
+        Errors = errors;
+    }
+}
+

src/Core/TakeoutSaaS.Shared.Abstractions/TakeoutSaaS.Shared.Abstractions.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+</Project>
+

src/Core/TakeoutSaaS.Shared.Abstractions/Tenancy/ITenantProvider.cs

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+namespace TakeoutSaaS.Shared.Abstractions.Tenancy;
+
+public interface ITenantProvider
+{
+    Guid GetCurrentTenantId();
+}
+