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/ecosystem.config.json

@@ -1,12 +1,13 @@
 {
   "apps": [{
     "name": "portfolio-backend",
-    "script": "server.js",
+    "script": "dotnet",
+    "args": "JoaoLoureiro.Portfolio.Api/bin/Release/net8.0/JoaoLoureiro.Portfolio.Api.dll",
     "instances": 1,
     "autorestart": true,
     "watch": false,
     "env": {
-      "NODE_ENV": "production"
+      "ASPNETCORE_ENVIRONMENT": "Production"
     }
   }]
 }