Enhanced Cloudflare integration and asynchronous custom domains

Bolder now supports self-service Cloudflare accounts alongside managed CDN integrations, allowing merchants to configure their own API tokens for automated cache purging. Custom domain verification and SSL certificate generation now run asynchronously in the background, and domain validation fully supports Cloudflare-proxied setups and custom CNAME targets. Additionally, cache-sweeping and file tasks have been moved to dedicated background workers to improve request performance and reliability.