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.Api/Validators/ContactMessageRequestValidator.cs

@@ -0,0 +1,16 @@
+using FluentValidation;
+using JoaoLoureiro.Portfolio.Api.Dtos;
+
+namespace JoaoLoureiro.Portfolio.Api.Validators;
+
+public class ContactMessageRequestValidator : AbstractValidator<ContactMessageRequest>
+{
+    public ContactMessageRequestValidator()
+    {
+        RuleFor(x => x.Name).NotEmpty().WithErrorCode("status_error_all_fields");
+        RuleFor(x => x.Message).NotEmpty().WithErrorCode("status_error_all_fields");
+        RuleFor(x => x.Email)
+            .NotEmpty().WithErrorCode("status_error_all_fields")
+            .EmailAddress().WithErrorCode("status_error_invalid_email");
+    }
+}