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>

docs/backlog/README.md

@@ -57,7 +57,7 @@ for its existing violations, so every WP ends green.
 | [WP-15](WP-15-missing-stories.md)       | Missing stories: shell + brief components                    | 3 · Storybook               | todo   |
 | [WP-16](WP-16-component-a11y.md)        | Component a11y: description wiring + alert role              | 4 · a11y                    | todo   |
 | [WP-17](WP-17-app-a11y.md)              | App-level a11y: route focus, template lint, WCAG checklist   | 4 · a11y                    | todo   |
-| [WP-18](WP-18-abac-capability-spine.md) | ABAC capability spine (Principal + capabilities, phase P1)   | 5 · productie-volwassenheid | todo   |
+| [WP-18](WP-18-abac-capability-spine.md) | ABAC capability spine (Principal + capabilities, phase P1)   | 5 · productie-volwassenheid | done   |
 | [WP-19](WP-19-e2e-smoke.md)             | Playwright e2e smoke                                         | 5 · productie-volwassenheid | todo   |
 | [WP-20](WP-20-second-locale.md)         | Second locale proof                                          | 5 · productie-volwassenheid | todo   |
 | [WP-21](WP-21-resilience-seams.md)      | Resilience seams (correlation-id, idempotency, retry)        | 5 · productie-volwassenheid | todo   |