Classifies each signup's email domain (domain-only, GDPR-friendly) alongside
the existing IP scrutiny signal, with matching held-welcome and auto
email_domain_block behavior. A report against an account with a flagged
domain suspends immediately (no reporter-count threshold) and blocks the
domain, classified live from the report payload rather than any signup-time
record so it also covers pre-existing accounts. Ships CHECK_MAIL_DRY_RUN=true
by default, independent of ABUSE_DRY_RUN, so the new report-triggered suspend
path stays inert until watched.