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.Tests/RegistrationTests.cs

@@ -121,4 +121,100 @@ public class RegistrationTests
         Assert.Contains("only an INGEDIEND", ex.Message);
         Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
     }
+
+    [Fact]
+    public void Taking_a_registration_into_behandeling_moves_it_to_in_behandeling()
+    {
+        var registration = Registration.Submit("123456782");
+
+        registration.TakeIntoBehandeling();
+
+        Assert.Equal(RegistrationStatus.InBehandeling, registration.Status);
+    }
+
+    [Fact]
+    public void Re_taking_an_in_behandeling_registration_is_idempotent()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.TakeIntoBehandeling();
+
+        registration.TakeIntoBehandeling();
+
+        Assert.Equal(RegistrationStatus.InBehandeling, registration.Status);
+    }
+
+    [Fact]
+    public void Taking_a_decided_registration_into_behandeling_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+        registration.Approve();
+
+        var ex = Assert.Throws<InvalidOperationException>(() => registration.TakeIntoBehandeling());
+        Assert.Contains("only an INGEDIEND", ex.Message);
+        Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
+    }
+
+    [Fact]
+    public void Approving_an_in_behandeling_registration_with_a_zaak_sets_it_ingeschreven()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+        registration.TakeIntoBehandeling();
+
+        registration.Approve();
+
+        Assert.Equal(RegistrationStatus.Ingeschreven, registration.Status);
+    }
+
+    [Fact]
+    public void Rejecting_a_registration_sets_it_afgewezen()
+    {
+        var registration = Registration.Submit("123456782");
+
+        registration.Reject();
+
+        Assert.Equal(RegistrationStatus.Afgewezen, registration.Status);
+    }
+
+    [Fact]
+    public void Rejecting_needs_no_zaak()
+    {
+        // A registration can be turned down before its zaak is ever opened, so Reject must not require one.
+        var registration = Registration.Submit("123456782");
+
+        registration.Reject();
+
+        Assert.Equal(RegistrationStatus.Afgewezen, registration.Status);
+        Assert.Null(registration.ZaakUrl);
+    }
+
+    [Fact]
+    public void Rejecting_an_in_behandeling_registration_sets_it_afgewezen()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.TakeIntoBehandeling();
+
+        registration.Reject();
+
+        Assert.Equal(RegistrationStatus.Afgewezen, registration.Status);
+    }
+
+    [Fact]
+    public void Deciding_an_already_decided_registration_is_rejected()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.Reject();
+
+        var approveEx = Assert.Throws<InvalidOperationException>(() =>
+        {
+            registration.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+            registration.Approve();
+        });
+        Assert.Contains("IN_BEHANDELING", approveEx.Message);
+
+        var rejectEx = Assert.Throws<InvalidOperationException>(() => registration.Reject());
+        Assert.Contains("Afgewezen", rejectEx.Message);
+        Assert.Equal(RegistrationStatus.Afgewezen, registration.Status);
+    }
 }