Prevents the form being abused as an open email relay to arbitrary
addresses (which is the realistic abuse vector — disk fill, server
compromise, and device exploitation are all bounded already).
Default allowlist: kindle.com, pbsync.com. Suffix matching so
subdomains (e.g. free.kindle.com) are also accepted.
Configurable via ALLOWED_READER_DOMAINS env var so the list can be
extended without code changes.
Hetzner (and most VPS providers) block outbound 465 by default as
anti-spam policy. Port 587 with STARTTLS is open and supported by
Resend (and any standard SMTP service). The transporter code already
flips `secure: false` automatically when port != 465, so no code
change is needed — just the default.
