Some small improvements for clarity.

* PostModel -> PostDto
* 404 test case
* README

service/Program.cs

@@ -35,7 +35,7 @@ app.UseRequestTimeouts();
 app.MapGet("/posts/{id}", async (AppDbContext dbContext, JsonPlaceholderClient client, int id) =>
 {
     // TODO: (Phase 1) Implement the logic to retrieve a post by ID and store it in the database.
-    // Consider some minimal error handling in case the post is not found (e.g. Id > 100).
+    // Consider some minimal error handling in case the post is not found (e.g. when testing with an Id > 100).
     return Results.Ok();
 })
 .WithRequestTimeout(TimeSpan.FromSeconds(29))