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/HealthCheck/SmtpHealthCheck.cs

@@ -0,0 +1,31 @@
+
+using JoaoLoureiro.Portfolio.Infrastructure.Settings;
+using MailKit.Net.Smtp;
+using MailKit.Security;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Microsoft.Extensions.Options;
+
+namespace JoaoLoureiro.Portfolio.Infrastructure.HealthCheck;
+
+public class SmtpHealthCheck(IOptions<SmtpSettings> smtpSettings) : IHealthCheck
+{
+    private readonly SmtpSettings _smtpSettings = smtpSettings.Value;
+
+    public async Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context,
+        CancellationToken cancellationToken = default)
+    {
+        using var client = new SmtpClient();
+        try
+        {
+            await client.ConnectAsync(_smtpSettings.Host, _smtpSettings.Port, SecureSocketOptions.StartTlsWhenAvailable, cancellationToken);
+            await client.AuthenticateAsync(_smtpSettings.User, _smtpSettings.Pass, cancellationToken);
+            await client.DisconnectAsync(true, cancellationToken);
+            return HealthCheckResult.Healthy("SMTP server is responding correctly.");
+        }
+        catch (Exception ex)
+        {
+            return HealthCheckResult.Unhealthy("Failed to connect or authenticate with SMTP server.", exception: ex);
+        }
+    }
+}