feat(domain): expose POST /registrations and the read endpoint (refs #6)

The BIG Domain Service Api wires the use cases and the hosted job worker:
POST /registrations creates the aggregate and starts the registratie process,
returning 202 with a location; GET /registrations/{id} reads the aggregate so
the eventually-opened zaak URL can be observed (ADR-0009). The Workflow Client
is registered once behind both Flowable ports; the ACL client and in-memory
store complete the wiring. Verified against a live flowable-rest: submit starts
a parked process and the worker polls it.

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

register-referentie.slnx

@@ -11,6 +11,7 @@
     <Project Path="services/domain/Big.Domain/Big.Domain.csproj" />
     <Project Path="services/domain/Big.Application/Big.Application.csproj" />
     <Project Path="services/domain/Big.Infrastructure/Big.Infrastructure.csproj" />
+    <Project Path="services/domain/Big.Api/Big.Api.csproj" />
     <Project Path="services/domain/Big.Tests/Big.Tests.csproj" />
   </Folder>
   <Folder Name="/services/bff/">

services/domain/Big.Api/Big.Api.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <ItemGroup>
+    <ProjectReference Include="..\Big.Application\Big.Application.csproj" />
+    <ProjectReference Include="..\Big.Infrastructure\Big.Infrastructure.csproj" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+</Project>

services/domain/Big.Api/Program.cs

@@ -0,0 +1,64 @@
+using Big.Application;
+using Big.Domain;
+using Big.Infrastructure;
+
+var builder = WebApplication.CreateBuilder(args);
+
+// Options bound from configuration (compose sets Flowable__* and Acl__* env vars).
+builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
+    .GetSection("Flowable").Get<FlowableOptions>()
+    ?? throw new InvalidOperationException("Missing configuration section 'Flowable'"));
+builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
+    .GetSection("Acl").Get<AclOptions>()
+    ?? throw new InvalidOperationException("Missing configuration section 'Acl'"));
+
+// The in-memory registration store is shared between the submit endpoint and the worker (ADR-0009).
+builder.Services.AddSingleton<IRegistrationStore, InMemoryRegistrationStore>();
+
+// The Workflow Client is one type behind two ports (start side + worker side); both resolve to the
+// same HttpClient-backed implementation — the only code that talks to Flowable (§8.2).
+builder.Services.AddHttpClient<FlowableWorkflowClient>();
+builder.Services.AddTransient<IWorkflowClient>(sp => sp.GetRequiredService<FlowableWorkflowClient>());
+builder.Services.AddTransient<IExternalWorkerClient>(sp => sp.GetRequiredService<FlowableWorkflowClient>());
+builder.Services.AddHttpClient<IAclClient, AclHttpClient>();
+
+builder.Services.AddScoped<SubmitRegistration>();
+builder.Services.AddScoped<OpenZaakWorker>();
+builder.Services.AddScoped<OpenZaakJobProcessor>();
+
+// The hosted external-task job worker polls Flowable and drives OpenZaakAanmaken to completion.
+builder.Services.AddHostedService<OpenZaakJobPump>();
+
+var app = builder.Build();
+
+app.MapGet("/health", () => "Healthy");
+
+// Submit a registration. The aggregate is created (INGEDIEND) and the registratie process started;
+// the zaak is opened later, off the request path, by the worker — so this returns 202 Accepted with
+// a location to read the registration's progress (ADR-0009, eventual consistency).
+app.MapPost("/registrations", async (SubmitRegistrationRequest body, SubmitRegistration submit, CancellationToken ct) =>
+{
+    var id = await submit.HandleAsync(new SubmitRegistrationCommand(body.Bsn), ct);
+    return Results.Accepted($"/registrations/{id}", new RegistrationResponse(id.ToString(), RegistrationStatus.Ingediend.ToString(), null));
+});
+
+// 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) =>
+{
+    if (!Guid.TryParse(id, out var guid))
+        return Results.NotFound();
+
+    var registration = await store.GetAsync(new RegistrationId(guid), ct);
+    return registration is null
+        ? Results.NotFound()
+        : Results.Ok(new RegistrationResponse(
+            registration.Id.ToString(), registration.Status.ToString(), registration.ZaakUrl?.ToString()));
+});
+
+await app.RunAsync();
+
+public sealed record SubmitRegistrationRequest(string Bsn);
+
+public sealed record RegistrationResponse(string RegistrationId, string Status, string? ZaakUrl);
+
+public partial class Program;

services/domain/Big.Api/appsettings.json

@@ -0,0 +1,9 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  },
+  "AllowedHosts": "*"
+}

services/domain/Big.slnx

@@ -2,5 +2,6 @@
   <Project Path="Big.Domain/Big.Domain.csproj" />
   <Project Path="Big.Application/Big.Application.csproj" />
   <Project Path="Big.Infrastructure/Big.Infrastructure.csproj" />
+  <Project Path="Big.Api/Big.Api.csproj" />
   <Project Path="Big.Tests/Big.Tests.csproj" />
 </Solution>