Standard.site publication email service

Chose a private newsletter service plus per-repo trigger surfaces over putting newsletter logic directly in the static blog repo because the repo can safely hold only the Astro form, Standard.site manifest client, and shared rendering helpers; Loops keys, webhook secrets, subscriber data, and send state must stay private. Rejected direct-to-Loops browser forms and repo-local state because they seem simpler for a GitHub Pages site, but they expose credentials and cannot safely own consent, idempotency, or retries. Email addresses must never appear in AT Protocol records or the public repo.