Pre-checked "InstantAdPay newsletter" opt-in on the join screen (read by both the
email-code and password signup paths). On new-account creation only, the server
silently subscribes them to the Sendy "InstantAdPay Newsletter" list
(boolean=true, opt-out always wins). New sendy.js helper reads the API key from
SENDY_API_KEY env or DATA_DIR/sendy.key on the volume (same pattern as
sendgrid.key); subscribe is fire-and-forget and never blocks signup.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
martbost