Add HSTS, gzip, Ntfy notifications, and paginate entries endpoint

- nginx: add HSTS (max-age=31536000) and gzip for text/css/js/json
- entries: add limit/offset query params (default 200, max 1000)
- Ntfy: wire NTFY_URL/NTFY_TOKEN through env; notify on startup, new
  registration, admin login, user disable, and user delete
- httpx==0.28.1 added for async Ntfy HTTP calls

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

backend/app/utils/notify.py

@@ -0,0 +1,19 @@
+import logging
+import httpx
+
+from app.config import settings
+
+logger = logging.getLogger("bourbonacci.notify")
+
+
+async def notify(title: str, message: str, priority: str = "default") -> None:
+    if not settings.ntfy_url:
+        return
+    headers = {"Title": title, "Priority": priority}
+    if settings.ntfy_token:
+        headers["Authorization"] = f"Bearer {settings.ntfy_token}"
+    try:
+        async with httpx.AsyncClient(timeout=5) as client:
+            await client.post(settings.ntfy_url, content=message, headers=headers)
+    except Exception as exc:
+        logger.warning("Ntfy notification failed: %s", exc)