feat(bff): add /health endpoint (refs #28)

Register ASP.NET Core health checks and map GET /health. The endpoint
returns 200 with a "Healthy" body by default, making the red test pass.
Smallest change to go green.

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

services/bff/Bff.Api/appsettings.Development.json

@@ -0,0 +1,8 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  }
+}