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>

src/app/shared/infrastructure/me.adapter.spec.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { parseMe } from './me.adapter';
+
+describe('parseMe (trust boundary)', () => {
+  it('parses a known capability list', () => {
+    const r = parseMe({ capabilities: ['brief:approve', 'brief:reject', 'brief:send'] });
+    expect(r).toEqual({ ok: true, value: ['brief:approve', 'brief:reject', 'brief:send'] });
+  });
+
+  it('parses an empty list (drafter — no capabilities)', () => {
+    expect(parseMe({ capabilities: [] })).toEqual({ ok: true, value: [] });
+  });
+
+  it('drops unrecognized capability strings instead of rejecting the response', () => {
+    const r = parseMe({ capabilities: ['brief:approve', 'unknown:future-thing'] });
+    expect(r).toEqual({ ok: true, value: ['brief:approve'] });
+  });
+
+  it('rejects malformed responses instead of trusting them', () => {
+    expect(parseMe(null).ok).toBe(false);
+    expect(parseMe({}).ok).toBe(false);
+    expect(parseMe({ capabilities: 'brief:approve' }).ok).toBe(false);
+  });
+});