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.ts

@@ -0,0 +1,33 @@
+import { Injectable, inject, resource } from '@angular/core';
+import { Result, ok, err } from '@shared/kernel/fp';
+import { Capability } from '@shared/domain/capability';
+import { ApiClient } from '@shared/infrastructure/api-client';
+
+const KNOWN: readonly Capability[] = ['brief:approve', 'brief:reject', 'brief:send'];
+
+/**
+ * Infrastructure adapter for `GET /me` (PRD-0002 §6): the current principal's
+ * coarse, role-derived capabilities — nav/menu-level, not tied to any one screen's
+ * live status (contrast a screen's own decision DTO, e.g. `BriefViewDto.decisions`).
+ */
+@Injectable({ providedIn: 'root' })
+export class MeAdapter {
+  private client = inject(ApiClient);
+
+  meResource() {
+    return resource({ loader: () => this.client.me() });
+  }
+}
+
+/**
+ * Trust-boundary parse. An unrecognized capability string is dropped rather than
+ * rejecting the whole response — deny-by-default already covers it (AccessStore.can
+ * returns false for anything not in the set), and it lets the backend grow the
+ * capability list without breaking an older FE build.
+ */
+export function parseMe(json: unknown): Result<string, Capability[]> {
+  if (typeof json !== 'object' || json === null) return err('me: not an object');
+  const dto = json as { capabilities?: unknown };
+  if (!Array.isArray(dto.capabilities)) return err('me: missing/invalid capabilities');
+  return ok(dto.capabilities.filter((c): c is Capability => KNOWN.includes(c as Capability)));
+}