Initial exercise setup.

Additions

* Instructions for manually starting the database.
* Each phase now has a success criteria with how to verify.
* Troubleshooting section for working with the Docker DB container.
* All phases of the exercise are described in README.md (with as little advanced hints as possible).
* DFD to illustrate phase 1
* Debugging starts the database container and applies schema migrations (no setup required).
* Launch settings for VS Code, VS 2022 and Rider (IntelliJ IDEA).
* Swagger UI is available at <http://localhost:8080/swagger/index.html> for testing.

service/JsonPlaceholderClient.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Net.Http;
+using System.Net.Http.Json;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace service;
+
+public class JsonPlaceholderClient
+{
+    private readonly HttpClient _client;
+
+    public JsonPlaceholderClient(HttpClient client)
+    {
+        _client = client;
+        // TODO: Configure the client as needed.
+    }
+
+    public async Task<PostModel?> GetPostByIdAsync(int id, CancellationToken ct = default)
+    {
+        // TODO: Implement the logic to call the external service and retrieve the post by ID.
+        throw new NotImplementedException("This method will be implemented in Phase 1.");
+    }
+}