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/Bff.Api.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+</Project>