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/">