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/Services/SmtpEmailSender.cs

@@ -0,0 +1,44 @@
+using JoaoLoureiro.Portfolio.Core.Interfaces;
+using JoaoLoureiro.Portfolio.Core.Models;
+using JoaoLoureiro.Portfolio.Infrastructure.Settings;
+using MailKit.Net.Smtp;
+using MailKit.Security;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using MimeKit;
+
+namespace JoaoLoureiro.Portfolio.Infrastructure.Services;
+
+public class SmtpEmailSender(IOptions<SmtpSettings> smtpSettings, ILogger<SmtpEmailSender> logger) : IEmailSender
+{
+    private readonly SmtpSettings _smtpSettings = smtpSettings.Value;
+    private readonly ILogger<SmtpEmailSender> _logger = logger;
+
+    public async Task SendEmailAsync(EmailToSend email, CancellationToken cancellationToken = default)
+    {
+        var mimeMessage = new MimeMessage();
+        var fromAddress = _smtpSettings.FromEmail ?? _smtpSettings.User;
+
+        mimeMessage.From.Add(new MailboxAddress(email.SenderName, fromAddress));
+        mimeMessage.To.Add(MailboxAddress.Parse(_smtpSettings.ReceivingEmail));
+        mimeMessage.ReplyTo.Add(MailboxAddress.Parse(email.ReplyToEmail));
+        mimeMessage.Subject = $"New Portfolio Contact: {email.SenderName}";
+
+        var builder = new BodyBuilder
+        {
+            TextBody = $"Name: {email.SenderName}\nEmail: {email.ReplyToEmail}\nMessage: {email.MessageBody}",
+            HtmlBody = $@"<p><strong>Name:</strong> {email.SenderName}</p>
+                        <p><strong>Email:</strong> <a href=""mailto:{email.ReplyToEmail}"">{email.ReplyToEmail}</a></p>
+                        <p><strong>Message:</strong></p>
+                        <p>{email.MessageBody.Replace("\n", "<br>")}</p>"
+        };
+        mimeMessage.Body = builder.ToMessageBody();
+
+        using var client = new SmtpClient();
+
+        await client.ConnectAsync(_smtpSettings.Host, _smtpSettings.Port, SecureSocketOptions.StartTlsWhenAvailable, cancellationToken);
+        await client.AuthenticateAsync(_smtpSettings.User, _smtpSettings.Pass, cancellationToken);
+        await client.SendAsync(mimeMessage, cancellationToken);
+        _logger.LogInformation("Email sent successfully to {Recipient}", _smtpSettings.ReceivingEmail);
+    }
+}