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>

.env.example

@@ -15,3 +15,7 @@ ADMIN_PASSWORD=changeme_admin
 
 # CORS: comma-separated list of allowed origins
 ALLOWED_ORIGINS=https://yourdomain.com
+
+# Ntfy push notifications (optional — leave blank to disable)
+NTFY_URL=https://ntfy.example.com/your-topic
+NTFY_TOKEN=