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/Program.cs

@@ -0,0 +1,122 @@
+using AutoMapper;
+using FluentValidation;
+using HealthChecks.UI.Client;
+using JoaoLoureiro.Portfolio.Api.Dtos;
+using JoaoLoureiro.Portfolio.Core.Interfaces;
+using JoaoLoureiro.Portfolio.Core.Models;
+using JoaoLoureiro.Portfolio.Infrastructure.HealthCheck;
+using JoaoLoureiro.Portfolio.Infrastructure.Services;
+using JoaoLoureiro.Portfolio.Infrastructure.Settings;
+using Microsoft.AspNetCore.Diagnostics.HealthChecks;
+using Microsoft.AspNetCore.HttpOverrides;
+using Microsoft.AspNetCore.Mvc;
+using Serilog;
+using System.Net;
+
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Host.UseSerilog((context, config) => config.ReadFrom.Configuration(context.Configuration));
+
+builder.Services.AddProblemDetails();
+builder.Services.AddEndpointsApiExplorer();
+builder.Services.AddSwaggerGen();
+
+var corsOrigins = builder.Configuration["CorsOrigins"];
+builder.Services.AddCors(options =>
+{
+    options.AddDefaultPolicy(policy =>
+    {
+        if (corsOrigins is not null)
+        {
+            policy.WithOrigins(corsOrigins.Split(','))
+                  .AllowAnyHeader()
+                  .AllowAnyMethod();
+        }
+    });
+});
+
+builder.Services.AddOptions<SmtpSettings>()
+    .Bind(builder.Configuration.GetSection("SmtpSettings"))
+    .ValidateDataAnnotations()
+    .ValidateOnStart();
+
+builder.Services.AddScoped<IEmailSender, SmtpEmailSender>();
+builder.Services.AddValidatorsFromAssemblyContaining<Program>();
+
+builder.Services.AddAutoMapper(cfg => { }, typeof(Program));
+builder.Services.AddHealthChecks()
+    .AddCheck<SmtpHealthCheck>("SMTP");
+
+builder.Services.Configure<ForwardedHeadersOptions>(options =>
+{
+    options.KnownProxies.Add(IPAddress.Parse(builder.Configuration["ProxyIP"] ?? "10.0.10.10"));
+});
+
+var app = builder.Build();
+
+app.UseForwardedHeaders(new ForwardedHeadersOptions
+{
+    ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto
+});
+
+app.UseSerilogRequestLogging();
+app.UseExceptionHandler();
+app.UseStatusCodePages();
+
+if (app.Environment.IsDevelopment())
+{
+    app.UseSwagger();
+    app.UseSwaggerUI();
+}
+
+app.UseHttpsRedirection();
+app.UseCors();
+
+
+app.MapHealthChecks("/health", new HealthCheckOptions
+{
+    ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse
+});
+
+app.MapPost("/api/email/send", async (
+    [FromBody] ContactMessageRequest request,
+    IValidator<ContactMessageRequest> validator,
+    IEmailSender emailSender,
+    IMapper mapper,
+    ILogger<Program> logger,
+    CancellationToken cancellationToken) =>
+{
+    var validationResult = await validator.ValidateAsync(request, cancellationToken);
+    if (!validationResult.IsValid)
+    {
+        var error = validationResult.Errors.First();
+        return Results.BadRequest(new { errorKey = error.ErrorCode });
+    }
+
+    try
+    {
+        var email = mapper.Map<EmailToSend>(request);
+
+        await emailSender.SendEmailAsync(email, cancellationToken);
+
+        return Results.Ok(new { message = "Message sent successfully!" });
+    }
+    catch (Exception ex)
+    {
+        logger.LogError(ex, "An unexpected error occurred while sending email.");
+        return Results.Problem(
+            detail: "An unexpected error occurred on the server.",
+            statusCode: 500,
+            title: "Server Error",
+            extensions: new Dictionary<string, object?> { { "errorKey", "server_unexpected_error" } }
+        );
+    }
+})
+.WithName("SendContactMessage")
+.WithDescription("Accepts a contact form submission and sends it as an email.")
+.Produces(StatusCodes.Status200OK)
+.Produces<HttpValidationProblemDetails>(StatusCodes.Status400BadRequest)
+.Produces<ProblemDetails>(StatusCodes.Status500InternalServerError);
+
+
+app.Run();