feat(brief): WP-18 — ABAC capability spine (PRD-0002 phase P1)

Replace the FE-computed authorization anti-pattern in BriefStore.editable
(derived from the unverified X-Role header) with server-computed decision
flags, mirroring the existing HerregistratieDecisionsDto pattern:

- Backend: Authz.cs is the single authorization helper — the SAME check
  (Authz.CanActOn) both gates BriefStore.Review's mutations and computes
  the BriefDecisionsDto flags shipped on every brief response, so emit
  and enforce can never drift. New GET /me returns coarse, role-derived
  capabilities (PRD-0002 SS6).
- Every brief endpoint (including send, previously ungated on HttpContext)
  now returns a fresh BriefViewDto so decisions never go stale after a
  mutation.
- FE: brief.store.ts reads canEdit/canApprove/canReject/canSend off the
  loaded decisions instead of computing them from currentRole(); the
  brief.machine carries decisions through every status transition.
- New shared/domain/capability.ts + shared/application/access.store.ts +
  shared/infrastructure/me.adapter.ts: the general capability-spine
  infrastructure (AccessStore.can(), capabilityGuard) for future routes.

Deviates from the original WP-18 draft by NOT renaming auth/domain's
Session to a Principal union — ADR-0002 explicitly defers that refactor
until a second actor exists, and the brief workflow's drafter/approver
identity turned out to be a separate axis from the SSP login session
entirely. See docs/backlog/WP-18-abac-capability-spine.md for the full
as-built record.

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

backend/tests/BigRegister.Tests/AuthzTests.cs

@@ -0,0 +1,68 @@
+using BigRegister.Domain.Authorization;
+using Xunit;
+
+namespace BigRegister.Tests;
+
+/// <summary>
+/// Unit tests for the single authorization helper (PRD-0002 phase P1) that both
+/// computes the brief screen's decision flags and gates the mutation endpoints.
+/// </summary>
+public class AuthzTests
+{
+  private static readonly Principal Drafter = new(PrincipalRole.Drafter);
+  private static readonly Principal Approver = new(PrincipalRole.Approver);
+  private const string DrafterId = "demo-drafter";
+
+  [Fact]
+  public void Drafter_may_not_approve_or_reject_even_when_submitted()
+  {
+    Assert.False(Authz.CanActOn(BriefAction.Approve, Drafter, DrafterId));
+    Assert.False(Authz.CanActOn(BriefAction.Reject, Drafter, DrafterId));
+  }
+
+  [Fact]
+  public void Approver_may_approve_and_reject_a_different_drafters_letter()
+  {
+    Assert.True(Authz.CanActOn(BriefAction.Approve, Approver, DrafterId));
+    Assert.True(Authz.CanActOn(BriefAction.Reject, Approver, DrafterId));
+  }
+
+  [Fact]
+  public void Send_is_not_role_gated()
+  {
+    Assert.True(Authz.CanActOn(BriefAction.Send, Drafter, DrafterId));
+    Assert.True(Authz.CanActOn(BriefAction.Send, Approver, DrafterId));
+  }
+
+  [Theory]
+  [InlineData("draft")]
+  [InlineData("rejected")]
+  public void Decisions_CanEdit_true_for_drafter_in_editable_statuses(string status)
+  {
+    Assert.True(Authz.Decisions(Drafter, status, DrafterId).CanEdit);
+    Assert.False(Authz.Decisions(Approver, status, DrafterId).CanEdit);
+  }
+
+  [Fact]
+  public void Decisions_CanApprove_requires_submitted_status_and_approver_role()
+  {
+    Assert.True(Authz.Decisions(Approver, "submitted", DrafterId).CanApprove);
+    Assert.False(Authz.Decisions(Approver, "draft", DrafterId).CanApprove);
+    Assert.False(Authz.Decisions(Drafter, "submitted", DrafterId).CanApprove);
+  }
+
+  [Fact]
+  public void Decisions_CanSend_requires_approved_status_only()
+  {
+    Assert.True(Authz.Decisions(Drafter, "approved", DrafterId).CanSend);
+    Assert.True(Authz.Decisions(Approver, "approved", DrafterId).CanSend);
+    Assert.False(Authz.Decisions(Approver, "submitted", DrafterId).CanSend);
+  }
+
+  [Fact]
+  public void RoleCapabilities_are_empty_for_drafter_and_the_three_brief_capabilities_for_approver()
+  {
+    Assert.Empty(Authz.RoleCapabilities(Drafter));
+    Assert.Equal(new[] { "brief:approve", "brief:reject", "brief:send" }, Authz.RoleCapabilities(Approver));
+  }
+}