feat(#13): S-12a — beoordeling decision model (domain) (#82)

## What & why

First sub-slice of **S-12 (#13)** — the **beoordeling decision model** in the Domain Service. Foundation for the behandel-portal: it gives the domain a proper decision lifecycle before any UI/Flowable/BFF work.

- **Statuses:** add `InBehandeling` and `Afgewezen` to `RegistrationStatus`.
- **Aggregate:** `TakeIntoBehandeling()` (`Ingediend → InBehandeling`, idempotent, guards terminal states); generalise the behandelaar decision — `Approve()` (requires a zaak) and new `Reject()` both act on an `Ingediend`/`InBehandeling` registration → `Ingeschreven`/`Afgewezen`.
- **Use-case:** `BeoordeelRegistratie` (`goedkeuren` sets the zaak's final status via the ACL §8.1 → `Ingeschreven`; `afwijzen` → `Afgewezen`, domain-only for now). Idempotent.
- **Endpoint:** `POST /registrations/{id}/decide` (`{ "besluit": "goedkeuren" | "afwijzen" }`), superseding the temporary `/approve` (retired when the portal lands, S-12d).
- **BDD:** `EenRegistratieBeoordelen.feature` — goedkeuren + afwijzen scenarios (feature-scoped bindings).

**Scoped out** to later S-12 sub-slices: Flowable user-task claim/complete + BPMN `userTask` (S-12b), BFF `/behandel/*` + medewerker authz (S-12c), the Angular behandel-portal + e2e (S-12d), and propagating a *rejection* to the zaak/projection via the ACL.

## Definition of Done

- [x] Linked issue: #13 (umbrella; this PR `refs`, does not close)
- [x] Tests first; red → green per behaviour
- [x] Unit + acceptance green (`make unit`): domain 65, acceptance 9
- [x] Mutation ≥ break(90): domain 98.77%, no survivors in new code (the one unkilled mutant is the pre-existing `FlowableWorkflowClient` baseline)
- [ ] CI green (pending)

Part of #13.

Reviewed-on: #82

services/domain/Big.Application/BeoordeelRegistratie.cs

@@ -0,0 +1,57 @@
+using Big.Domain;
+
+namespace Big.Application;
+
+/// <summary>A behandelaar's beoordeling outcome, in domain language.</summary>
+public enum BeoordelingsBesluit
+{
+    /// <summary>Approve — enter the registration in the register.</summary>
+    Goedkeuren,
+
+    /// <summary>Reject — turn the registration down.</summary>
+    Afwijzen,
+}
+
+/// <summary>A behandelaar's decision on a registration.</summary>
+public sealed record BeoordeelRegistratieCommand(RegistrationId RegistrationId, BeoordelingsBesluit Besluit);
+
+/// <summary>
+/// The beoordeling use case (S-12): apply a behandelaar's decision to a registration.
+/// <see cref="BeoordelingsBesluit.Goedkeuren"/> sets the zaak's final status via the ACL (§8.1) and
+/// advances the aggregate to INGESCHREVEN; <see cref="BeoordelingsBesluit.Afwijzen"/> advances it to
+/// AFGEWEZEN in the domain (propagating a rejection to the zaak, so the openbaar projection reflects
+/// it, is a later sub-slice of S-12). Both decisions are idempotent — a repeated or redelivered
+/// decision that matches the current terminal state is a no-op, so the ACL is not called twice.
+/// </summary>
+public sealed class BeoordeelRegistratie(IRegistrationStore store, IAclClient acl)
+{
+    public async Task HandleAsync(BeoordeelRegistratieCommand command, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(command);
+
+        var registration = await store.GetAsync(command.RegistrationId, ct)
+            ?? throw new InvalidOperationException($"No registration {command.RegistrationId} to decide.");
+
+        switch (command.Besluit)
+        {
+            case BeoordelingsBesluit.Goedkeuren:
+                // A repeated approval is a no-op: don't set the zaak status a second time.
+                if (registration.Status == RegistrationStatus.Ingeschreven)
+                    return;
+                if (registration.ZaakUrl is null)
+                    throw new InvalidOperationException(
+                        $"Registration {command.RegistrationId} has no zaak yet; it cannot be approved.");
+                await acl.ApproveZaakAsync(registration.ZaakUrl, ct);
+                registration.Approve();
+                break;
+
+            case BeoordelingsBesluit.Afwijzen:
+                if (registration.Status == RegistrationStatus.Afgewezen)
+                    return;
+                registration.Reject();
+                break;
+        }
+
+        await store.SaveAsync(registration, ct);
+    }
+}