Add ntfy push notifications for security-relevant events

Sends alerts on admin login, new registrations, user disable/delete, and
impersonation. NTFY_URL and NTFY_TOKEN are optional — leave blank to disable.

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

docker-compose.yml

@@ -54,6 +54,8 @@ services:
       ADMIN_USERNAME:   ${ADMIN_USERNAME}
       ADMIN_PASSWORD:   ${ADMIN_PASSWORD}
       JWT_SECRET:       ${JWT_SECRET}
+      NTFY_URL:         ${NTFY_URL:-}
+      NTFY_TOKEN:       ${NTFY_TOKEN:-}
     healthcheck:
       test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8000/api/health')"]
       interval: 30s