Phase 5: platform feature-flags CRUD, impersonation, billing catalogue

Three new platform screens on top of the Phase 1-4 work.

Feature flags (/feature-flags) — platform-wide flag registry. New route +
lib/arcadia/feature-flags.ts, capability platform.feature_flags, nav under
Automation. List/create/edit/delete with a per-row default toggle; pairs with
the Phase-4 per-tenant override tab.

Impersonation — "Impersonate" action on active users. Entirely client-side
token swap in session.ts (beginImpersonation parks the operator's session +
API token and swaps to the impersonation token; endImpersonation restores it),
with a sticky "Viewing as <email> — Stop" banner in the shell driven by the
JWT's impersonated_by claim. Stop is client-side because the impersonation
token carries the target's roles and can't reach the admin-gated /stop
endpoint; impersonation is stateless JWT so restoring the parked token is
sufficient.

Billing (/billing) — replaced the coming-soon stub with the real plan
catalogue from GET /billing/plans (lib/arcadia/billing.ts). Per-tenant plan
assignment stays on the tenant detail page; Entitlements + Apps remain honestly
marked "Soon".

Verified in-browser with real backend; typecheck adds zero errors (36→36).

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

app/routes.ts

@@ -29,6 +29,7 @@ export default [
   route("status-page", "routes/status-page.tsx"),
   route("search", "routes/search.tsx"),
   route("billing", "routes/billing.tsx"),
+  route("feature-flags", "routes/feature-flags.tsx"),
   route("integrations", "routes/integrations.tsx"),
   // CREMA:ROUTES
 ] satisfies RouteConfig