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>

nginx/default.conf

@@ -2,9 +2,15 @@ server {
     listen 80;
     server_tokens off;
 
+    gzip on;
+    gzip_types text/plain text/css application/json application/javascript text/javascript;
+    gzip_min_length 1024;
+    gzip_vary on;
+
     root /usr/share/nginx/html;
     index index.html;
 
+    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
     add_header X-Frame-Options "SAMEORIGIN" always;
     add_header X-Content-Type-Options "nosniff" always;
     add_header X-XSS-Protection "1; mode=block" always;