feat(backend): Replace Node.js API with .NET 8

This commit completes the migration of the backend service from a Node.js/Express application to an ASP.NET Core 8 Minimal API.

- Re-implemented all API endpoints in C# for improved performance and type safety.
- Updated the Gitea Actions workflow to use the `setup-dotnet` action, `dotnet publish` for building, and now caches NuGet packages.
- Modified the deployment to create an `appsettings.Production.json` file from Gitea secrets instead of a `.env` file.
- Updated the PM2 ecosystem configuration to run the application using the `dotnet` interpreter.

backend/JoaoLoureiro.Portfolio.Domain/Entities/ContactMessage.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace JoaoLoureiro.Portfolio.Domain.Entities
+{
+    public class ContactMessage(string name, Email replyToEmail, string message)
+    {
+        public string Name { get; private set; } = name;
+        public Email ReplyToEmail { get; private set; } = replyToEmail;
+        public string Message { get; private set; } = message;
+    }
+}