email: add plaintext MIME part + stable Message-ID hostname

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.

docker-compose.yml

@@ -300,6 +300,11 @@ services:
 
   listmonk:
     image: listmonk/listmonk:latest
+    # Stable hostname so the Message-ID Listmonk derives from the container OS
+    # hostname is perfwest.performancewest.net, NOT the random docker container
+    # id -> @localhost.localdomain (a spam-score signal; see deliverability
+    # runbook). Matches Listmonk's SMTP hello_hostname.
+    hostname: perfwest.performancewest.net
     ports:
       - "9100:9000"
     environment:
@@ -335,6 +340,9 @@ services:
   # mynetworks 172.16/12). host.docker.internal is mapped for convenience.
   listmonk-hc:
     image: listmonk/listmonk:latest
+    # Stable hostname -> Message-ID @perfwest.performancewest.net, not the random
+    # container id -> @localhost.localdomain (spam-score signal). See listmonk above.
+    hostname: perfwest.performancewest.net
     ports:
       - "9101:9000"
     extra_hosts: