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/services/rmd_filing.py

@@ -136,6 +136,8 @@ class RMDFilingHandler(BaseServiceHandler):
                     import smtplib
                     from email.mime.text import MIMEText
                     from email.mime.multipart import MIMEMultipart
+
+                    from scripts._email_plaintext import html_to_text
                     subject = f"Action Required — Review your RMD certification for {entity.get('legal_name', order_number)}"
                     body = (
                         f"<h2>Your RMD Certification is Ready for Review</h2>"
@@ -161,6 +163,7 @@ class RMDFilingHandler(BaseServiceHandler):
                         msg["From"] = smtp_from
                         msg["To"] = customer_email
                         msg["Reply-To"] = "info@performancewest.net"
+                        msg.attach(MIMEText(html_to_text(body), "plain"))
                         msg.attach(MIMEText(body, "html"))
                         with smtplib.SMTP(smtp_host, smtp_port) as server:
                             server.starttls()