test(domain): herregistratie inscription date + reminder-due rule (refs #18)

The Registration aggregate gains the herregistratie clock: Approve now records
the inscription moment, from which the herregistratie deadline and a
reminder-due rule are derived. Members are stubbed so the suite compiles and the
new assertions fail; the green commit implements them. Approve's signature gains
the inscription moment (callers now supply 'now' from an injected TimeProvider).

refs #18

services/domain/Big.Api/Program.cs

@@ -15,6 +15,10 @@ builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
 // The in-memory registration store is shared between the submit endpoint and the worker (ADR-0009).
 builder.Services.AddSingleton<IRegistrationStore, InMemoryRegistrationStore>();
 
+// The system clock, injected wherever a use case needs "now" (e.g. stamping the inscription moment
+// on approval, S-17). Injected as TimeProvider so tests can substitute a fixed clock.
+builder.Services.AddSingleton(TimeProvider.System);
+
 // 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>();