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.Infrastructure/Settings/SmtpSettings.cs

@@ -0,0 +1,11 @@
+namespace JoaoLoureiro.Portfolio.Infrastructure.Settings;
+
+public class SmtpSettings
+{
+    public required string Host { get; set; }
+    public int Port { get; set; }
+    public required string User { get; set; }
+    public required string Pass { get; set; }
+    public string? FromEmail { get; set; }
+    public required string ReceivingEmail { get; set; }
+}