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>

api/.env.example

@@ -55,14 +55,12 @@ MAIL_FROM_ADDRESS="noreply@crewli.app"
 MAIL_FROM_NAME="${APP_NAME}"
 
 # CORS + Sanctum — SPA origins (no trailing slash; must match the browser URL)
-FRONTEND_ADMIN_URL=http://localhost:5173
 FRONTEND_APP_URL=http://localhost:5174
 FRONTEND_PORTAL_URL=http://localhost:5175
-SANCTUM_STATEFUL_DOMAINS=localhost:5173,localhost:5174,localhost:5175
+SANCTUM_STATEFUL_DOMAINS=localhost:5174,localhost:5175
 
-# --- Production (crewli.app) — uncomment and adjust hostnames if you use this layout:
+# --- Production (crewli.app) — uncomment and adjust hostnames:
 # APP_URL=https://api.crewli.app
-# FRONTEND_ADMIN_URL=https://admin.crewli.app
-# FRONTEND_APP_URL=https://app.crewli.app
+# FRONTEND_APP_URL=https://crewli.app
 # FRONTEND_PORTAL_URL=https://portal.crewli.app
-# SANCTUM_STATEFUL_DOMAINS=admin.crewli.app,app.crewli.app,portal.crewli.app
+# SANCTUM_STATEFUL_DOMAINS=crewli.app,portal.crewli.app