test(bff): endpoints, JWT auth and public-safe projection (refs #8)

Failing tests for the BFF walking-skeleton endpoints:
- POST /self-service/registrations rejects missing/malformed/wrong-key/expired
  tokens (401) and, with a valid digid token, forwards the bsn to the domain and
  returns 202 (WebApplicationFactory + a local test signing key, ADR-0010).
- GET /openbaar/register serves public-safe rows anonymously (never the bsn) and
  filters by q.
- OpenbaarProjection.PublicView (pure) filters by id and maps to id+status only.

Endpoints and PublicView are stubs so the tests compile and fail on their
assertions; the green commit implements them.

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

services/bff/Bff.Api/Program.cs

@@ -1,10 +1,44 @@
+using Bff.Api;
+using Microsoft.AspNetCore.Authentication.JwtBearer;
+
 var builder = WebApplication.CreateBuilder(args);
+
+var keycloakAuthority = builder.Configuration["Keycloak:Authority"]
+    ?? throw new InvalidOperationException("Missing configuration 'Keycloak:Authority'");
+var domainBaseUrl = builder.Configuration["Downstream:Domain:BaseUrl"]
+    ?? throw new InvalidOperationException("Missing configuration 'Downstream:Domain:BaseUrl'");
+var projectionBaseUrl = builder.Configuration["Downstream:Projection:BaseUrl"]
+    ?? throw new InvalidOperationException("Missing configuration 'Downstream:Projection:BaseUrl'");
+
+// Validate Keycloak-issued tokens (ADR-0010). Audience validation is off for the walking skeleton —
+// Keycloak's audience mapping is a later hardening; signature/issuer/expiry are validated.
+builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
+    .AddJwtBearer(options =>
+    {
+        options.Authority = keycloakAuthority;
+        options.RequireHttpsMetadata = false;
+        options.TokenValidationParameters.ValidateAudience = false;
+    });
+builder.Services.AddAuthorization();
+
+// The BFF is the portals' only backend; it fans out to the domain and projection (§8.3).
+builder.Services.AddHttpClient<IDomainClient, DomainClient>(c => c.BaseAddress = new Uri(domainBaseUrl));
+builder.Services.AddHttpClient<IProjectionClient, ProjectionClient>(c => c.BaseAddress = new Uri(projectionBaseUrl));
+
 builder.Services.AddHealthChecks();
+builder.Services.AddOpenApi();
 
 var app = builder.Build();
 
-app.MapGet("/", () => "BFF placeholder");
+app.UseAuthentication();
+app.UseAuthorization();
+
 app.MapHealthChecks("/health");
+app.MapOpenApi();
+
+// STUB (red): no auth requirement, no downstream call, no filtering.
+app.MapPost("/self-service/registrations", () => Results.Ok());
+app.MapGet("/openbaar/register", (string? q) => Results.Ok(Array.Empty<OpenbaarEntry>()));
 
 app.Run();