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/Dockerfile

@@ -0,0 +1,36 @@
+# Stage 1: Build the application
+# Use the .NET SDK image which contains all the tools to build and publish
+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
+WORKDIR /src
+
+# Copy all .csproj files and the .sln file first. This is a key optimization.
+# Docker will cache this layer, and will only re-run `dotnet restore` if a project file changes.
+COPY ["JoaoLoureiro.Portfolio.Core/JoaoLoureiro.Portfolio.Core.csproj", "JoaoLoureiro.Portfolio.Core/"]
+COPY ["JoaoLoureiro.Portfolio.Infrastructure/JoaoLoureiro.Portfolio.Infrastructure.csproj", "JoaoLoureiro.Portfolio.Infrastructure/"]
+COPY ["JoaoLoureiro.Portfolio.Api/JoaoLoureiro.Portfolio.Api.csproj", "JoaoLoureiro.Portfolio.Api/"]
+COPY ["JoaoLoureiro.Portfolio.sln", "."]
+
+# Restore all NuGet packages for the entire solution
+RUN dotnet restore "JoaoLoureiro.Portfolio.sln"
+
+# Copy the rest of the source code
+COPY . .
+
+# Publish the API project, creating the release-ready artifacts
+WORKDIR "JoaoLoureiro.Portfolio.Api"
+RUN dotnet publish "JoaoLoureiro.Portfolio.Api.csproj" -c Release -o /app/publish --no-restore
+
+# Stage 2: Create the final, minimal runtime image
+# Use the lightweight ASP.NET runtime image, which is much smaller than the SDK
+FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS final
+WORKDIR /app
+
+# The new .NET 8 images run as a non-root 'app' user by default for better security.
+# We expose port 8080, as the default internal port 80 is often a privileged port.
+EXPOSE 8080
+
+# Copy the published output from the build stage
+COPY --from=build /app/publish .
+
+# Set the entrypoint for the container
+ENTRYPOINT ["dotnet", "JoaoLoureiro.Portfolio.Api.dll"]