feat(admin): runtime feature flags (catalog-in-code, admin toggle, FE+backend)

Catalog declared in code (Domain/Features/FeatureFlags.cs, build-validated), on/off state
persisted in SQLite (FeatureFlagStore + migration). GET /flags (drives FE gating) + admin
PUT /admin/flags/{key} (new flags:manage capability + FlagsAdmin gate). Enforced end-to-end:
the `inschrijving-open` flag hides the Inschrijven nav item + dashboard action (FE) AND makes
POST /applications for a registratie 403 when off (backend). FE FeatureFlagStore mirrors
AccessStore (enabled() deny-by-default); admin toggle page at /beheer/functies in ADMIN_LINKS.
+4 backend tests, /me cap-list updated, client regenerated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/layout/admin-links.ts

@@ -37,4 +37,10 @@ export const ADMIN_LINKS: readonly AdminLink[] = [
     to: '/beheer/audit',
     cap: 'cases:manage',
   },
+  {
+    label: $localize`:@@header.nav.functies:Functievlaggen`,
+    description: $localize`:@@admin.link.functies.desc:Functionaliteit aan- of uitzetten`,
+    to: '/beheer/functies',
+    cap: 'flags:manage',
+  },
 ];