Add ntfy authentication support (username/password and API key)

- Settings table gets ntfy_username, ntfy_password, ntfy_api_key columns
- Backend applies Basic or Bearer auth header when sending notifications
- Settings page UI lets you toggle between no auth, basic, or token auth
- Masked credential display on load to avoid exposing stored secrets
- README updated with auth modes documentation

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

nginx/html/css/style.css

@@ -628,3 +628,9 @@ a:hover { text-decoration: underline; }
   .main-content { margin-left: 0; padding: 1rem; }
   .stats-row { grid-template-columns: 1fr 1fr; }
 }
+
+/* Ntfy auth toggle */
+.hidden { display: none !important; }
+.auth-toggle { display: flex; gap: 1.25rem; flex-wrap: wrap; }
+.auth-toggle-option { display: flex; align-items: center; gap: 0.35rem; cursor: pointer; font-size: 0.9rem; }
+.auth-toggle-option input[type="radio"] { accent-color: var(--green-mid); }