Parallelize independent wordpress-setup tasks

[retlehs]

This experiments with task-level parallelism in wordpress-setup for independent per-site work:

• run local database creation asynchronously per site, then wait before DB grants
• run self-signed certificate generation asynchronously per site, then wait before cleanup and Nginx config
• build a unique client certificate download list and download client certs asynchronously
• keep DB user grants, PHP-FPM config, and Nginx config serialized to avoid shared-resource conflicts

The goal is to reduce provision time for projects with multiple sites or SSL/client-cert work without changing the observable provisioning order of shared services

Notes

This intentionally avoids role-level parallelism. wordpress-setup still waits before touching shared Nginx/PHP-FPM state, and DB grants remain serial because multiple sites may share a DB user.

Async behavior is controlled by:

• wordpress_setup_async_timeout
• wordpress_setup_async_poll_delay
• wordpress_setup_async_retries

Validation Needed

• Run before/after benchmarks for local development provisioning
• Run before/after benchmarks for production/server provisioning
• Test against a fresh local VM
• Test against a fresh remote server
• Confirm repeat provisioning remains idempotent
• Confirm multi-site configs behave correctly, especially with local DBs and self-signed SSL