统一服务端点架构，支持多端接口与数据库切换

重构项目结构，引入 Avalonia-Common、Avalonia-EFCore、Avalonia-Services，实现 API 与桌面端统一端点注册、过滤器、鉴权和标准响应格式。支持多数据库自动迁移与配置，集成 Serilog 日志系统。移除旧路由与控制器，提升接口一致性与可维护性。

Avalonia-Services/Models/UserEntity.cs

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+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Avalonia_Services.Models
+{
+    /// <summary>
+    /// 用户实体 —— 演示数据库 CRUD 操作。
+    /// </summary>
+    [Table("Users")]
+    public class UserEntity
+    {
+        [Key]
+        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+        public int Id { get; set; }
+
+        [MaxLength(100)]
+        public string? Name { get; set; }
+
+        [MaxLength(200)]
+        public string? Email { get; set; }
+
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+    }
+}

Avalonia-Services/Models/WeatherForecastEntity.cs

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+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Avalonia_Services.Models
+{
+    /// <summary>
+    /// 天气预报数据实体 —— 对应数据库中的 WeatherForecasts 表。
+    /// </summary>
+    [Table("WeatherForecasts")]
+    public class WeatherForecastEntity
+    {
+        [Key]
+        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+        public int Id { get; set; }
+
+        public DateOnly Date { get; set; }
+
+        public int TemperatureC { get; set; }
+
+        [MaxLength(200)]
+        public string? Summary { get; set; }
+
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+    }
+}