feat(domain): take a registration into behandeling; generalise decisions (refs #13)

Adds InBehandeling + Afgewezen statuses and TakeIntoBehandeling(). Generalises the
behandelaar decision: Approve() (requires a zaak) and new Reject() both act on an
INGEDIEND or IN_BEHANDELING registration, moving it to Ingeschreven/Afgewezen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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}).");
+    }
 }

services/domain/Big.Domain/RegistrationStatus.cs

@@ -1,13 +1,20 @@
 namespace Big.Domain;
 
-/// <summary>The lifecycle states a <see cref="Registration"/> moves through. The walking
-/// skeleton knows <see cref="Ingediend"/> and the terminal <see cref="Ingeschreven"/>; withdrawal,
-/// beoordeling and herregistratie states arrive in their own slices (Iteration 2+).</summary>
+/// <summary>The lifecycle states a <see cref="Registration"/> moves through. Submission starts in
+/// <see cref="Ingediend"/>; a behandelaar takes it <see cref="InBehandeling"/> and decides it into one
+/// of the terminal states <see cref="Ingeschreven"/> (approved) or <see cref="Afgewezen"/> (rejected).
+/// Withdrawal and herregistratie states arrive in their own slices (S-11+).</summary>
 public enum RegistrationStatus
 {
     /// <summary>Submitted by the zorgprofessional; the registratie process has been started.</summary>
     Ingediend,
 
-    /// <summary>Approved: entered in the register. Terminal in the walking skeleton (S-09b).</summary>
+    /// <summary>Picked up by a behandelaar for beoordeling (S-12).</summary>
+    InBehandeling,
+
+    /// <summary>Approved: entered in the register. Terminal.</summary>
     Ingeschreven,
+
+    /// <summary>Rejected by the behandelaar. Terminal.</summary>
+    Afgewezen,
 }