Send ARCs to beta readers

ARCs tab → "+ Invite". Each reader gets a tokenized link (/arc/<bookId>?t=…) that serves the latest EPUB or screen PDF with their email watermarked on every page. Opens are tracked. Tokens expire after 90 days; revoke any time. If RESEND_API_KEY is set, the platform emails them automatically — otherwise you copy the link out and send it yourself.