feat: add bootstrap admin auth docs and fix config loading

- Document bootstrap admin login via KMTN_ADMIN_USERNAME/PASSWORD
  env vars, including setup guide, curl examples, and config table
  entries across docker-compose and bare-metal deployment sections
- Add KMTN_JWT_SECRET_KEY to all deployment config examples and
  production checklist
- Fix docker-entrypoint.sh: remove unused API_UPSTREAM variable from
  config.json envsubst (only API_PUBLIC_URL is needed)
- Fix app-config.service: default apiBaseUrl now reads from Angular
  environment instead of empty string, and config.json path drops
  leading ./ for cleaner resolution

docker-entrypoint.sh

@@ -3,6 +3,6 @@
 set -e
 
 envsubst '${API_UPSTREAM}' < /etc/nginx/conf.d/nginx.conf.template > /etc/nginx/conf.d/default.conf
-envsubst '${API_UPSTREAM},${API_PUBLIC_URL}' < /usr/share/nginx/html/config.json.template > /usr/share/nginx/html/config.json
+envsubst '${API_PUBLIC_URL}' < /usr/share/nginx/html/config.json.template > /usr/share/nginx/html/config.json
 
 exec nginx -g 'daemon off;'