fix(privacy): mask the owner BSN on the cross-owner case lists (RB-03)

Mappers.ToAdminSummaryDto set Owner to the raw BSN. Both consumers are
cross-owner lists read by someone who is not the subject — GET /admin/cases
and GET /werkvoorraad — while GET /beoordeling/{id}, the detail view of the
same data, already masked it. The detail screen showed ******782 and the list
one click earlier showed the whole thing.

Masked in the mapper rather than at each endpoint, so a third cross-owner
list cannot be added that forgets to.

MaskTail moves out of Program.cs into Domain/People/Pii.cs: it now has
callers in Contracts, Program.cs and (once RB-04 lands) Data, and a second
hand-rolled copy is how one of them drifts into leaking. Documented as
idempotent, which is what lets /beoordeling/{id} keep its own call —
IZaakSource has a second implementation whose Owner is mapped from the
OpenZaak zaak identificatie, so that endpoint should not depend on which
source answered.

No frontend change: all three consumers display the value, and the parse
boundaries only require a non-empty string.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/BigRegister.Api/Contracts/Mappers.cs

@@ -70,8 +70,12 @@ public static class Mappers
       a.CreatedAt.ToString("o"), a.UpdatedAt.ToString("o"), SubmittedAtOf(a));
 
   /// Admin summary — same shape plus the owner (WP-36; the user-facing list leaves Owner null).
+  /// The owner is a BSN, and both consumers of this mapper are cross-owner lists read by
+  /// someone who is not the subject (`/admin/cases`, `/werkvoorraad`), so it goes out masked
+  /// (RB-03/BIO-003). Masking here rather than at each endpoint means a third cross-owner
+  /// list cannot be added that forgets to.
   public static ApplicationSummaryDto ToAdminSummaryDto(this Aanvraag a, DateTimeOffset now) =>
-      a.ToSummaryDto(now) with { Owner = a.Owner };
+      a.ToSummaryDto(now) with { Owner = Pii.MaskTail(a.Owner, 3) };
 
   public static ApplicationDetailDto ToDetailDto(this Aanvraag a, DateTimeOffset now) => new(
       a.Id, a.Type, a.ToStatusDto(now), DraftOf(a), a.DocumentIds,

backend/src/BigRegister.Api/Domain/People/Pii.cs

@@ -0,0 +1,18 @@
+namespace BigRegister.Domain.People;
+
+/// <summary>
+/// One redaction rule for identifiers that must not leave the server in full (BSN,
+/// BIG-nummer). Lives in <c>Domain/</c> because three layers need it — the DTO mappers
+/// (<c>Contracts/Mappers.cs</c>), the audit writes (<c>Data/DocumentStore.cs</c>) and the
+/// endpoints themselves — and a second hand-rolled copy is exactly how one of them drifts
+/// into leaking. Mirrors the FE <c>maskTail</c> (<c>libs/shared/src/ui/debug-state/mask.ts</c>)
+/// so wire redaction and the dev panel agree on what a masked value looks like.
+/// </summary>
+public static class Pii
+{
+  /// Keep the last <paramref name="keep"/> characters, mask the rest. Idempotent: masking an
+  /// already-masked value is a no-op, so a defence-in-depth second call is harmless.
+  public static string MaskTail(string value, int keep) =>
+      value.Length <= keep ? new string('*', value.Length)
+                           : new string('*', value.Length - keep) + value[^keep..];
+}

backend/src/BigRegister.Api/Program.cs

@@ -12,6 +12,7 @@ using BigRegister.Domain.Documents;
 using BigRegister.Domain.Features;
 using BigRegister.Domain.Intake;
 using BigRegister.Domain.Letters;
+using BigRegister.Domain.People;
 using BigRegister.Domain.Registrations;
 using BigRegister.Domain.Submissions;
 using BigRegister.Api.Zgw;
@@ -460,7 +461,10 @@ api.MapGet("/beoordeling/{id}", (string id, HttpContext ctx, IZaakSource zaken)
       if (c is null || c.Status.Tag == "Concept") return Results.NotFound();
       var docs = DocumentStore.ByIds(c.DocumentIds)
           .Select(d => new BeoordelingDocumentDto(d.DocumentId, d.CategoryId, d.FileName)).ToList();
-      var masked = c with { Owner = MaskTail(c.Owner!, 3) };
+      // Belt and braces: ToAdminSummaryDto already masks the local source (RB-03) and
+      // MaskTail is idempotent, but IZaakSource has a second implementation whose Owner
+      // is mapped from OpenZaak, so this stays as the guarantee for this response.
+      var masked = c with { Owner = Pii.MaskTail(c.Owner!, 3) };
       // WP-68 (F3): non-throwing — c.Status.Tag crosses the IZaakSource wire boundary, so an
       // unrecognised tag degrades to "cannot decide" instead of a 500.
       var canBesluiten = Enum.TryParse<AanvraagStatusTag>(c.Status.Tag, out var tag) && BeoordelingRules.CanDecide(tag);
@@ -869,12 +873,6 @@ void RecordZgwDivergence(HttpContext ctx, string id, string referentie, Exceptio
   AuthzAuditStore.Record("zgw:divergence", referentie, allowed: false, Authz.ResolvePrincipal(ctx).Role.ToString(), cid);
 }
 
-// Keep the last `keep` characters, mask the rest — mirrors the FE maskTail
-// (src/app/shared/ui/debug-state/mask.ts) so wire redaction and the dev panel agree.
-static string MaskTail(string value, int keep) =>
-    value.Length <= keep ? new string('*', value.Length)
-                         : new string('*', value.Length - keep) + value[^keep..];
-
 static string Now() => DateTimeOffset.UtcNow.ToString("o");
 
 BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
@@ -888,7 +886,7 @@ BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
     // behandel scherm can show whom/what it concerns without brief/ importing registratie.
     // The BIG-nummer ships MASKED by default (PRD-0002 §5c, field-level PII); the reveal
     // endpoint returns the full value, gated + audited.
-    new CaseContextDto(SeedData.Registration.Naam, MaskTail(SeedData.Registration.BigNummer, 3), e.Beroep, BriefSeed.AanvraagReferentie));
+    new CaseContextDto(SeedData.Registration.Naam, Pii.MaskTail(SeedData.Registration.BigNummer, 3), e.Beroep, BriefSeed.AanvraagReferentie));
 
 // Emit (decision flags, via ToView) and enforce (Forbidden/Conflict below) both run
 // through Authz — see BriefStore.Review and Authz.CanActOn — so they cannot drift.