Refine queue worker pool behavior and surface worker configuration

[Its-donkey]

##Summary:

Refine queue delivery behavior and surface worker configuration.

##Changes

• send 554 5.7.1 before closing unauthorized sessions for clearer diagnostics
• persist queue storage atomically by rolling back partial recipient writes
• add configurable worker pool with idempotent shutdown and concurrency test
• expose SMTP_QUEUE_WORKERS across README, .env.example, installer, and site; document in changelog

##Checklist

• CHANGELOG.md updated
• Branch name follows type/section/kebab-feature
• Commits follow convention
• Push after each discrete change
• One PR per feature

##Testing

• GOTOOLCHAIN=local go test ./...

##Labels

• feature
• queue