Add comprehensive backend test suite and helper access

GameList.Tests/Support/TestClientExtensions.cs

@@ -43,4 +43,11 @@ internal static class TestClientExtensions
         var json = await response.Content.ReadFromJsonAsync<JsonElement>();
         return json.GetProperty("id").GetInt32();
     }
+
+    public static async Task<Guid> GetProfileIdAsync(this HttpClient client)
+    {
+        var me = await client.GetFromJsonAsync<JsonElement>("/api/me");
+        return Guid.Parse(me.GetProperty("id").GetString()!);
+    }
+
 }