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 <[email protected]>

backend/requirements.txt

@@ -10,3 +10,4 @@ pydantic-settings==2.5.2
 alembic==1.13.3
 python-multipart==0.0.22
 email-validator==2.2.0
+httpx==0.27.2