docs: remove admin SPA references and update production URLs

The admin SPA (apps/admin/) has been retired. Its functionality now
lives in apps/app/ under /platform/* routes for super_admin users.
Updated all documentation to reflect: 2 SPAs instead of 3, removed
FRONTEND_ADMIN_URL/port 5173 references, changed production URL from
app.crewli.app to crewli.app. Retired admin-specific security audit
findings (A13-2, A13-4, A13-5, A13-7) and APPS-01 backlog item.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev-docs/BACKLOG.md

@@ -351,15 +351,9 @@ voor third-party integraties (ticketing, HR, etc.)
 
 ## Apps & Platforms
 
-### APPS-01 — apps/admin/ volledig bouwen
+### ~~APPS-01 — apps/admin/ volledig bouwen~~ RETIRED
 
-**Aanleiding:** Super Admin panel voor platform-beheer.
-**Wat:**
-
-- Alle organisaties beheren
-- Billing status wijzigen
-- Platform-gebruikers beheren
-- Usage statistieken
+**Status:** Retired — admin SPA (`apps/admin/`) is afgeschaft. Super admin functionaliteit is verplaatst naar `apps/app/` onder `/platform/*` routes voor `super_admin` gebruikers.
 
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