Two deliverability hardening fixes from the email audit:
1. Plaintext (altbody): all campaigns were HTML-only. Listmonk only emits
multipart/alternative when altbody is set, and HTML-only bulk mail is a
spam-score signal. New scripts/_email_plaintext.py renders a readable
text/plain part from the HTML body (dependency-free; preserves Listmonk
{{ .Subscriber }}/{{ UnsubscribeURL }} template tags, turns links into
'text (url)'). Wired into the trucking builder (and thus UCR + IFTA, which
reuse create_and_schedule_campaign) and the healthcare builder.
2. Stable container hostname: Listmonk derived its Message-ID from the random
docker container id -> @localhost.localdomain (spam-score signal). Pin both
listmonk + listmonk-hc hostname to perfwest.performancewest.net, matching
Listmonk's SMTP hello_hostname.
Part of the email-deliverability incident hardening.
