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>

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>