feat(domain): expose POST /registrations/{id}/decide for the beoordeling (refs #13)

The behandel-portal's decision reaches the domain here (via the BFF, later slices):
goedkeuren/afwijzen, idempotent, superseding the temporary /approve endpoint.

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

services/domain/Big.Api/Program.cs

@@ -24,6 +24,7 @@ builder.Services.AddHttpClient<IAclClient, AclHttpClient>();
 
 builder.Services.AddScoped<SubmitRegistration>();
 builder.Services.AddScoped<ApproveRegistration>();
+builder.Services.AddScoped<BeoordeelRegistratie>();
 builder.Services.AddScoped<OpenZaakWorker>();
 builder.Services.AddScoped<OpenZaakJobProcessor>();
 
@@ -55,6 +56,22 @@ app.MapPost("/registrations/{id}/approve", async (string id, ApproveRegistration
     return Results.NoContent();
 });
 
+// The behandelaar's beoordeling (S-12): decide a registration goedkeuren (→ INGESCHREVEN, sets the
+// zaak's final status via the ACL) or afwijzen (→ AFGEWEZEN). Idempotent. This is the domain contract
+// the behandel-portal's decision reaches through the BFF; it supersedes the temporary /approve above,
+// which is retired once the portal lands.
+app.MapPost("/registrations/{id}/decide", async (string id, DecideRequest body, BeoordeelRegistratie beoordeel, CancellationToken ct) =>
+{
+    if (!Guid.TryParse(id, out var guid))
+        return Results.NotFound();
+
+    if (!Enum.TryParse<BeoordelingsBesluit>(body.Besluit, ignoreCase: true, out var besluit))
+        return Results.BadRequest(new { error = $"Unknown besluit '{body.Besluit}'. Expected 'goedkeuren' or 'afwijzen'." });
+
+    await beoordeel.HandleAsync(new BeoordeelRegistratieCommand(new RegistrationId(guid), besluit), ct);
+    return Results.NoContent();
+});
+
 // Read a registration. Its zaak URL appears once the worker has opened the zaak (eventually).
 app.MapGet("/registrations/{id}", async (string id, IRegistrationStore store, CancellationToken ct) =>
 {
@@ -72,6 +89,8 @@ await app.RunAsync();
 
 public sealed record SubmitRegistrationRequest(string Bsn);
 
+public sealed record DecideRequest(string Besluit);
+
 public sealed record RegistrationResponse(string RegistrationId, string Status, string? ZaakUrl);
 
 public partial class Program;