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.Application/Contracts/ICallTokenService.cs

@@ -0,0 +1,9 @@
+namespace Ping.Application.Contracts
+{
+    public record CallAccessToken(string Jwt, string RoomName);
+
+    public interface ICallTokenService
+    {
+        CallAccessToken CreateAccessToken(string roomName, string identity, string? displayName);
+    }
+}