Add Ntfy push notifications for super admin events

Sends alerts to a configurable Ntfy topic on: new user registration,
account lockout after 5 failed login attempts, and login attempts on an
already-locked account. Fire-and-forget — never raises if Ntfy is down.

Configure via NTFY_URL and NTFY_TOKEN in .env.

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

docker-compose.yml

@@ -33,6 +33,8 @@ services:
       CORS_ORIGINS: ${CORS_ORIGINS:-http://localhost:8057}
       ADMIN_USERNAME: ${ADMIN_USERNAME}
       ADMIN_PASSWORD: ${ADMIN_PASSWORD}
+      NTFY_URL: ${NTFY_URL:-}
+      NTFY_TOKEN: ${NTFY_TOKEN:-}
     depends_on:
       db:
         condition: service_healthy