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.Domain/Registration.cs

@@ -66,10 +66,28 @@ public sealed class Registration
     }
 
     /// <summary>
-    /// Approve the registration — the behandelaar's decision to enter it in the register. Advances
-    /// <see cref="RegistrationStatus.Ingediend"/> → <see cref="RegistrationStatus.Ingeschreven"/>.
-    /// Requires an opened zaak (the approval sets that zaak's status via the ACL), and only a
-    /// submitted registration can be approved: re-approving is rejected as an invalid transition.
+    /// A behandelaar picks the registration up for beoordeling. Advances
+    /// <see cref="RegistrationStatus.Ingediend"/> → <see cref="RegistrationStatus.InBehandeling"/>.
+    /// Re-taking one already <see cref="RegistrationStatus.InBehandeling"/> is a no-op (the same or
+    /// another behandelaar re-opens it); a decided registration can no longer be taken into behandeling.
+    /// </summary>
+    public void TakeIntoBehandeling()
+    {
+        if (Status == RegistrationStatus.InBehandeling)
+            return;
+
+        if (Status != RegistrationStatus.Ingediend)
+            throw new InvalidOperationException(
+                $"Registration {Id} is {Status}; only an INGEDIEND registration can be taken into behandeling.");
+
+        Status = RegistrationStatus.InBehandeling;
+    }
+
+    /// <summary>
+    /// Approve the registration — the behandelaar's decision to enter it in the register. Advances a
+    /// submitted or in-behandeling registration to <see cref="RegistrationStatus.Ingeschreven"/>.
+    /// Requires an opened zaak (the approval sets that zaak's status via the ACL); a registration that
+    /// has already been decided cannot be approved again.
     /// </summary>
     public void Approve()
     {
@@ -77,10 +95,27 @@ public sealed class Registration
             throw new InvalidOperationException(
                 $"Registration {Id} has no zaak yet; it cannot be approved before its zaak is opened.");
 
-        if (Status != RegistrationStatus.Ingediend)
-            throw new InvalidOperationException(
-                $"Registration {Id} is {Status}; only an INGEDIEND registration can be approved.");
-
+        RequireOpenForDecision(nameof(Approve));
         Status = RegistrationStatus.Ingeschreven;
     }
+
+    /// <summary>
+    /// Reject the registration — the behandelaar's decision not to enter it in the register. Advances a
+    /// submitted or in-behandeling registration to <see cref="RegistrationStatus.Afgewezen"/>. Unlike
+    /// approval this needs no zaak: a registration can be rejected before or after its zaak is opened.
+    /// A registration that has already been decided cannot be rejected again.
+    /// </summary>
+    public void Reject()
+    {
+        RequireOpenForDecision(nameof(Reject));
+        Status = RegistrationStatus.Afgewezen;
+    }
+
+    // A decision is only valid while the registration is still open (INGEDIEND or IN_BEHANDELING).
+    private void RequireOpenForDecision(string decision)
+    {
+        if (Status is not (RegistrationStatus.Ingediend or RegistrationStatus.InBehandeling))
+            throw new InvalidOperationException(
+                $"Registration {Id} is {Status}; only an INGEDIEND or IN_BEHANDELING registration can be decided ({decision}).");
+    }
 }