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.Application/HerregistratieReminderSweep.cs

@@ -0,0 +1,18 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>
+/// The herregistratie reminder sweep (S-17): find the inscriptions whose herregistratie deadline is
+/// within the reminder window and have not yet been reminded, mark each reminded, and persist it. Pure
+/// application logic over ports — it knows nothing of Quartz; the scheduled job that fires it on a cron
+/// lives in Infrastructure (mirroring how the pumps' processors are pure and the pump is the shell).
+/// Idempotent: <see cref="Registration.MarkHerregistratieReminderVerstuurd"/> drops an inscription from
+/// the next sweep's candidate set, so a re-fire reminds no one twice. Returns the reminded ids so the
+/// caller can observe the sweep's effect — the reminder itself is the flag persisted on the aggregate.
+/// </summary>
+public sealed class HerregistratieReminderSweep(IRegistrationStore store, TimeProvider clock)
+{
+    public Task<IReadOnlyList<RegistrationId>> SweepAsync(CancellationToken ct = default)
+        => Task.FromResult<IReadOnlyList<RegistrationId>>([]); // RED stub
+}