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>

docker-compose.yml

@@ -21,6 +21,8 @@ services:
       - ADMIN_USERNAME=${ADMIN_USERNAME}
       - ADMIN_PASSWORD=${ADMIN_PASSWORD}
       - ALLOWED_ORIGINS=${ALLOWED_ORIGINS:-http://localhost}
+      - NTFY_URL=${NTFY_URL:-}
+      - NTFY_TOKEN=${NTFY_TOKEN:-}
     depends_on:
       db:
         condition: service_healthy