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/Users/GetUserById.cs

@@ -0,0 +1,22 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Ping.Application.Contracts;
+using Ping.Application.Users.Responses;
+
+namespace Ping.Application.Users
+{
+    public record GetUserByIdQuery(long Id) : IQuery<ErrorOr<UserResponse>>;
+
+    public class GetUserByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetUserByIdQuery, ErrorOr<UserResponse>>
+    {
+        public async ValueTask<ErrorOr<UserResponse>> Handle(GetUserByIdQuery request, CancellationToken cancellationToken)
+        {
+            var user = await database.Users.AsNoTracking().FirstOrDefaultAsync(u => u.Id == request.Id, cancellationToken);
+            return user is null
+                ? Error.NotFound(description: $"User {request.Id} not found")
+                : UserResponse.FromEntity(user);
+        }
+    }
+}