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/fcc_carrier_registration.py

@@ -282,6 +282,8 @@ class FCCCarrierRegistrationHandler:
             from email.mime.text import MIMEText
             from email.mime.multipart import MIMEMultipart
 
+            from scripts._email_plaintext import html_to_text
+
             email = order.get("customer_email", "")
             name = order.get("customer_name", "")
             if not email:
@@ -302,6 +304,7 @@ class FCCCarrierRegistrationHandler:
             msg["Subject"] = subject
             msg["From"] = os.environ.get("SMTP_FROM", "Performance West <noreply@performancewest.net>")
             msg["To"] = email
+            msg.attach(MIMEText(html_to_text(body), "plain"))
             msg.attach(MIMEText(body, "html"))
 
             smtp_host = os.environ.get("SMTP_HOST", "co.carrierone.com")