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/Mapping/MappingProfile.cs

@@ -0,0 +1,15 @@
+using AutoMapper;
+using JoaoLoureiro.Portfolio.Api.Dtos;
+using JoaoLoureiro.Portfolio.Core.Models;
+
+namespace JoaoLoureiro.Portfolio.Api.Mapping;
+
+public class MappingProfile : Profile
+{
+    public MappingProfile() {
+        CreateMap<ContactMessageRequest, EmailToSend>()
+        .ForMember(dest => dest.SenderName, opt => opt.MapFrom(src => src.Name))
+        .ForMember(dest => dest.ReplyToEmail, opt => opt.MapFrom(src => src.Email))
+        .ForMember(dest => dest.MessageBody, opt => opt.MapFrom(src => src.Message));
+    }
+}