test(domain): herregistratie reminder sweep flags + returns due inscriptions (refs #18)

HerregistratieReminderSweep reminds every due inscription, persists the flag so a
re-fire is a no-op, and returns the reminded ids. Stubbed to an empty result so
the new tests fail; green implements the sweep. Adds a FixedClock TimeProvider
fake so the sweep is deterministic without a testing package.

refs #18

services/domain/Big.Tests/Fakes.cs

@@ -90,6 +90,13 @@ internal sealed class FakeUserTaskClient(IReadOnlyList<BeoordelingTask> open) :
     }
 }
 
+/// <summary>A <see cref="TimeProvider"/> pinned to a fixed instant, so time-based use cases (the
+/// herregistratie sweep, S-17) are deterministic without the TimeProvider.Testing package.</summary>
+internal sealed class FixedClock(DateTimeOffset now) : TimeProvider
+{
+    public override DateTimeOffset GetUtcNow() => now;
+}
+
 /// <summary>A fake ACL client that records the bsn it was asked to open a zaak for and returns a
 /// fixed zaak URL.</summary>
 internal sealed class FakeAclClient(Uri? zaakUrl = null) : IAclClient