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.Application/Interfaces/IEmailSender.cs

@@ -0,0 +1,8 @@
+using JoaoLoureiro.Portfolio.Core.Models;
+
+namespace JoaoLoureiro.Portfolio.Core.Interfaces;
+
+public interface IEmailSender
+{
+    Task SendEmailAsync(EmailToSend email, CancellationToken cancellationToken = default);
+}

backend/JoaoLoureiro.Portfolio.Application/JoaoLoureiro.Portfolio.Core.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

backend/JoaoLoureiro.Portfolio.Application/Models/EmailToSend.cs

@@ -0,0 +1,8 @@
+namespace JoaoLoureiro.Portfolio.Core.Models;
+
+public class EmailToSend
+{
+    public required string SenderName { get; set; }
+    public required string ReplyToEmail { get; set; }
+    public required string MessageBody { get; set; }
+}