fix(email): add text/plain part to every transactional + telecom email

All transactional/worker senders built multipart/alternative (or mixed)
messages with ONLY an HTML part. A single-part multipart/alternative is
malformed and HTML-only mail is a spam-score signal -- the same class of
deliverability bug that hurt the campaign pipeline, but on the telecom /
filing / customer-transactional path (499-Q reminders, RMD/FCC filing
review links, intake/completion/delivery emails, commissions, etc).

- worker_email.send_worker_email: auto-derive plaintext from HTML when
  caller omits text= (fixes the shared helper for all current+future use)
- 16 rolled-their-own senders in scripts/workers/** + scripts/formation/
  document_delivery.py: attach html_to_text(...) plaintext sibling before
  the HTML part (job_server + document_delivery wrap text+html in an
  alternative sub-part so PDFs still attach to the mixed root)
- api/src/email.ts: add dependency-free htmlToText() and default
  sendEmail text to it (fixes checkout/webhook HTML-only sends)

Verified: all py files compile + import at runtime, api tsc passes,
htmlToText handles hrefs/lists/entities, 11 plaintext unit tests pass.
Telecom campaign 407 (Jun 8) was HTML-only + sent in the DKIM-broken
window -> 384 sent / 0 clicks (same junked-mail signature).

scripts/workers/usf_factor_monitor.py

@@ -52,6 +52,8 @@ from datetime import date, datetime
 from decimal import Decimal, InvalidOperation
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
+
+from scripts._email_plaintext import html_to_text
 from typing import Iterable
 
 import psycopg2
@@ -604,6 +606,7 @@ def send_email(*, to_email: str, subject: str, html_body: str, dry_run: bool) ->
     msg["From"] = FROM_EMAIL
     msg["To"] = to_email
     msg["Bcc"] = ADMIN_EMAIL
+    msg.attach(MIMEText(html_to_text(html_body), "plain"))
     msg.attach(MIMEText(html_body, "html"))
     try:
         with smtplib.SMTP(SMTP_HOST, SMTP_PORT) as server: