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/completion_emails.py

@@ -18,6 +18,8 @@ from datetime import datetime, timedelta, timezone
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
 
+from scripts._email_plaintext import html_to_text
+
 import psycopg2
 
 LOG = logging.getLogger("workers.completion_emails")
@@ -37,6 +39,7 @@ def send_email(to: str, subject: str, html: str):
     msg["From"] = f"Performance West <{SMTP_FROM}>"
     msg["To"] = to
     msg["Subject"] = subject
+    msg.attach(MIMEText(html_to_text(html), "plain"))
     msg.attach(MIMEText(html, "html"))
 
     import os as _smtp_os