test(bff): /health returns 200 Healthy (refs #28)

Add the BFF Api/Tests skeleton (.NET 10, ASP.NET Core minimal API, xUnit
+ WebApplicationFactory) and a failing test asserting GET /health returns
200 with a Healthy body. The endpoint does not exist yet — the test fails
with NotFound, establishing red before the implementation.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/bff/Bff.Api/Program.cs

@@ -0,0 +1,9 @@
+var builder = WebApplication.CreateBuilder(args);
+var app = builder.Build();
+
+app.MapGet("/", () => "BFF placeholder");
+
+app.Run();
+
+// Exposed so the test host (WebApplicationFactory<Program>) can boot the app.
+public partial class Program;