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/appsettings.json

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