feat: 新增配额包/支付相关实体与迁移

App：新增 operation_logs/quota_packages/tenant_payments/tenant_quota_package_purchases 表

Identity：修正 Avatar 字段类型（varchar(256)->text），保持现有数据不变

src/Application/TakeoutSaaS.Application/App/Subscriptions/Commands/ChangeSubscriptionPlanCommand.cs

@@ -0,0 +1,34 @@
+using MediatR;
+using System.ComponentModel.DataAnnotations;
+using TakeoutSaaS.Application.App.Subscriptions.Dto;
+
+namespace TakeoutSaaS.Application.App.Subscriptions.Commands;
+
+/// <summary>
+/// 变更套餐命令。
+/// </summary>
+public sealed record ChangeSubscriptionPlanCommand : IRequest<SubscriptionDetailDto?>
+{
+    /// <summary>
+    /// 订阅 ID（从路由参数绑定）。
+    /// </summary>
+    [Required]
+    public long SubscriptionId { get; init; }
+
+    /// <summary>
+    /// 目标套餐 ID。
+    /// </summary>
+    [Required]
+    public long TargetPackageId { get; init; }
+
+    /// <summary>
+    /// 是否立即生效，否则在下周期生效。
+    /// </summary>
+    public bool Immediate { get; init; }
+
+    /// <summary>
+    /// 备注信息。
+    /// </summary>
+    [MaxLength(500)]
+    public string? Notes { get; init; }
+}