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>

backend/app/utils/ntfy.py

@@ -0,0 +1,25 @@
+import logging
+import httpx
+from app.config import get_settings
+
+logger = logging.getLogger("homeschool.ntfy")
+
+
+async def notify(title: str, message: str, priority: str = "default", tags: list[str] | None = None) -> None:
+    """Fire-and-forget notification to Ntfy. Silently logs errors — never raises."""
+    settings = get_settings()
+    if not settings.ntfy_url:
+        return
+
+    headers = {"Title": title, "Priority": priority}
+    if tags:
+        headers["Tags"] = ",".join(tags)
+    if settings.ntfy_token:
+        headers["Authorization"] = f"Bearer {settings.ntfy_token}"
+
+    try:
+        async with httpx.AsyncClient(timeout=5) as client:
+            resp = await client.post(settings.ntfy_url, content=message, headers=headers)
+            resp.raise_for_status()
+    except Exception as exc:
+        logger.warning("Ntfy notification failed: %s", exc)