Fix critical and high security issues

- exec_script: relay now enforces admin role before forwarding to agent
- relay CORS: restrict allow_origins via ALLOWED_ORIGINS env var (docker-compose passes app URL)
- session-code: replace Math.random() with crypto.randomInt, add per-key rate limit (10 req/min)
- sessions GET: fix IDOR — users can only read their own sessions (admins see all)
- signal API: validate session ownership on create; enforce ownerUserId on all subsequent actions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -52,6 +52,7 @@ services:
       - "8765:8765"
     environment:
       DATABASE_URL: postgresql://${POSTGRES_USER:-remotelink}:${POSTGRES_PASSWORD}@postgres:5432/${POSTGRES_DB:-remotelink}
+      ALLOWED_ORIGINS: ${NEXT_PUBLIC_APP_URL:-http://localhost:3000}
     depends_on:
       postgres:
         condition: service_healthy