chore: remove admin SPA and update to two-app production setup

Remove apps/admin/ entirely — platform admin functionality now lives
in apps/app/ under /platform/* routes for super_admin users.

Production URL scheme changed:
- Organizer app: crewli.app (was app.crewli.app)
- Portal: portal.crewli.app (unchanged)
- API: api.crewli.app (unchanged)
- admin.crewli.app and app.crewli.app retired

Backend:
- Removed FRONTEND_ADMIN_URL config and admin cookie (crewli_admin_token)
  from SetAuthCookie, CookieBearerToken, cors.php, app.php
- Updated .env and .env.example (two origins, no port 5173)
- Updated cookie test: admin origin test → unknown origin fallback test

Infrastructure:
- Makefile: removed admin target
- deploy/nginx: updated CSP comment, removed admin vhost
- Updated README.md, CLAUDE.md, and all dev-docs

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

dev-docs/BACKLOG.md

@@ -130,7 +130,7 @@ Handmatige per-persoon toewijzing is de uitzondering, niet de norm.
 - Zoekbalk (CTRL+K) aansluiten op Crewli-entiteiten
   (personen, events, secties zoeken)
 - Notificatie bell koppelen aan COMM-01
-- App switcher: Organizer / Admin / Portal wisselen
+- App switcher: Organizer / Portal wisselen (admin SPA retired; platform admin in `/platform/*`)
 - User avatar: gekoppeld aan ingelogde gebruiker (deels al gedaan)
   **Prioriteit:** Middel — werkt zonder maar verbetert UX significant
 
@@ -410,7 +410,7 @@ De volgende items zijn geïmplementeerd en afgerond (673+ tests):
 - ~~Cross-event section auto-redirect~~ ✅
 - ~~Shift claiming in portal (5 endpoints, 26 tests, ClaimenTab + RoosterTab)~~ ✅
 - ~~Cross-app auth isolation (CookieBearerToken per app, 3 isolatietests)~~ ✅
-- ~~Password reset (alle 3 SPAs, custom notification, app-aware links)~~ ✅
+- ~~Password reset (beide SPAs, custom notification, app-aware links)~~ ✅
 - ~~Email change with verification (self-service + admin, 24h token expiry)~~ ✅
 - ~~Password change while logged in~~ ✅
 - ~~"Lid toevoegen als deelnemer" shortcut (2 endpoints, 11 tests)~~ ✅