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>

deploy/README.md

@@ -16,22 +16,10 @@ server {
 }
 ```
 
-### Organizer App (app.crewli.app)
+### Organizer App (crewli.app)
 ```nginx
 server {
-    server_name app.crewli.app;
-
-    include /path/to/deploy/nginx/security-headers.conf;
-    include /path/to/deploy/nginx/csp-spa.conf;
-
-    # ... rest of config
-}
-```
-
-### Admin (admin.crewli.app)
-```nginx
-server {
-    server_name admin.crewli.app;
+    server_name crewli.app;
 
     include /path/to/deploy/nginx/security-headers.conf;
     include /path/to/deploy/nginx/csp-spa.conf;
@@ -66,4 +54,4 @@ If using DirectAdmin with Nginx:
 1. Place the `.conf` files in `/usr/local/directadmin/data/users/USERNAME/nginx.conf`
    or use DirectAdmin's custom Nginx configuration feature
 2. Reload Nginx: `service nginx reload`
-3. Verify headers: `curl -I https://app.crewli.app | grep -i security`
+3. Verify headers: `curl -I https://crewli.app | grep -i security`