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/worker_email.py

@@ -17,10 +17,21 @@ from __future__ import annotations
 import logging
 import os
 import smtplib
+import sys
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
 from email.mime.application import MIMEApplication
 
+# Shared HTML -> plaintext converter (scripts/_email_plaintext.py). Adding a
+# text/plain part to every message is a deliverability win: a multipart/
+# alternative with ONLY an HTML part is malformed, and HTML-only mail is a
+# spam-score signal. Import is path-tolerant (module run from /app or scripts/).
+try:
+    from scripts._email_plaintext import html_to_text
+except ImportError:  # pragma: no cover - fallback when scripts/ is on sys.path
+    sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+    from _email_plaintext import html_to_text  # type: ignore
+
 LOG = logging.getLogger("workers.email")
 
 SMTP_HOST = os.getenv("SMTP_HOST", "co.carrierone.com")
@@ -53,8 +64,13 @@ def send_worker_email(
         msg["Subject"] = subject
 
         alt = MIMEMultipart("alternative")
-        if text:
-            alt.attach(MIMEText(text, "plain"))
+        # Always include a text/plain part. If the caller did not supply one,
+        # derive it from the HTML so the message is a well-formed multipart/
+        # alternative (HTML-only is malformed + a spam signal). RFC 2046: list
+        # the plain part first (least->most preferred).
+        plain = text if text else html_to_text(html)
+        if plain:
+            alt.attach(MIMEText(plain, "plain"))
         alt.attach(MIMEText(html, "html"))
         msg.attach(alt)