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/Properties/launchSettings.json

@@ -0,0 +1,52 @@
+{
+  "profiles": {
+    "http": {
+      "commandName": "Project",
+      "launchBrowser": true,
+      "launchUrl": "swagger",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      },
+      "dotnetRunMessages": true,
+      "applicationUrl": "http://localhost:5125"
+    },
+    "https": {
+      "commandName": "Project",
+      "launchBrowser": true,
+      "launchUrl": "swagger",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      },
+      "dotnetRunMessages": true,
+      "applicationUrl": "https://localhost:7153;http://localhost:5125"
+    },
+    "IIS Express": {
+      "commandName": "IISExpress",
+      "launchBrowser": true,
+      "launchUrl": "swagger",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    },
+    "Container (Dockerfile)": {
+      "commandName": "Docker",
+      "launchBrowser": true,
+      "launchUrl": "{Scheme}://{ServiceHost}:{ServicePort}/swagger",
+      "environmentVariables": {
+        "ASPNETCORE_HTTPS_PORTS": "8081",
+        "ASPNETCORE_HTTP_PORTS": "8080"
+      },
+      "publishAllPorts": true,
+      "useSSL": true
+    }
+  },
+  "$schema": "http://json.schemastore.org/launchsettings.json",
+  "iisSettings": {
+    "windowsAuthentication": false,
+    "anonymousAuthentication": true,
+    "iisExpress": {
+      "applicationUrl": "http://localhost:36910",
+      "sslPort": 44372
+    }
+  }
+}