Add Ping backend

.NET 10 solution for Ping, a Discord style chat app: Domain, Application
and Infrastructure projects plus the Api host. SignalR for chat and
presence, LiveKit for call tokens, EF Core on Postgres, S3 compatible
storage for attachments, and OIDC auth shared with the Musify Zitadel
instance. Includes the Compose stack for local development.

backend/Core/Ping.Domain/Entities/ServerMember.cs

@@ -0,0 +1,27 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using Ping.Domain.ValueObjects;
+
+namespace Ping.Domain.Entities
+{
+    [Table("ServerMember")]
+    public class ServerMember
+    {
+        public required Guid ServerId { get; set; }
+
+        public Server? Server { get; set; }
+
+        public required long UserId { get; set; }
+
+        public User? User { get; set; }
+
+        [MaxLength(64)]
+        public string? Nickname { get; set; }
+
+        [Required]
+        public ServerMemberRole Role { get; set; } = ServerMemberRole.Member;
+
+        [Required]
+        public DateTime JoinedAt { get; set; } = DateTime.UtcNow;
+    }
+}