feat(bff): committed OpenAPI contract + drift guard (refs #8)

Document typed responses (202 SubmitAccepted / 400 / 401 on self-service; 200
OpenbaarEntry[] on openbaar) so the generated spec carries real schemas for S-08's
client. A document transformer clears the auto-populated servers block so the spec
is host-independent and deterministic. Commit services/bff/openapi.json and add a
test asserting it matches the served /openapi/v1.json (fails on drift).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/bff/Bff.Api/Program.cs

@@ -27,7 +27,14 @@ builder.Services.AddHttpClient<IDomainClient, DomainClient>(c => c.BaseAddress =
 builder.Services.AddHttpClient<IProjectionClient, ProjectionClient>(c => c.BaseAddress = new Uri(projectionBaseUrl));
 
 builder.Services.AddHealthChecks();
-builder.Services.AddOpenApi();
+// Clear the auto-populated `servers` block so the committed spec is stable regardless of the host
+// the doc was generated from (the client sets its own base URL). Keeps the drift guard deterministic.
+builder.Services.AddOpenApi(options =>
+    options.AddDocumentTransformer((document, _, _) =>
+    {
+        document.Servers?.Clear();
+        return Task.CompletedTask;
+    }));
 
 var app = builder.Build();
 
@@ -47,14 +54,19 @@ app.MapPost("/self-service/registrations", async (ClaimsPrincipal user, IDomainC
 
     var accepted = await domain.SubmitRegistrationAsync(bsn, ct);
     return Results.Accepted($"/self-service/registrations/{accepted.RegistrationId}", accepted);
-}).RequireAuthorization();
+})
+    .RequireAuthorization()
+    .Produces<SubmitAccepted>(StatusCodes.Status202Accepted)
+    .Produces(StatusCodes.Status400BadRequest)
+    .Produces(StatusCodes.Status401Unauthorized);
 
 // Openbaar register: an anonymous public lookup that exposes only public-safe fields (S-09).
 app.MapGet("/openbaar/register", async (string? q, IProjectionClient projection, CancellationToken ct) =>
 {
     var entries = await projection.GetRegisterAsync(ct);
     return Results.Ok(OpenbaarProjection.PublicView(entries, q));
-});
+})
+    .Produces<IReadOnlyList<OpenbaarEntry>>(StatusCodes.Status200OK);
 
 app.Run();