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/formation/document_delivery.py

@@ -267,9 +267,13 @@ def _build_email(
     msg["Subject"] = f"Your {type_display} Has Been Filed — {entity_name}"
     msg["Reply-To"] = FROM_EMAIL
 
-    # HTML body
-    html_part = email.mime.text.MIMEText(html_body, "html", "utf-8")
-    msg.attach(html_part)
+    # HTML body wrapped in alternative with a plaintext sibling (HTML-only is
+    # malformed inside multipart + a spam signal); documents attach to mixed root.
+    from scripts._email_plaintext import html_to_text
+    alt = email.mime.multipart.MIMEMultipart("alternative")
+    alt.attach(email.mime.text.MIMEText(html_to_text(html_body), "plain", "utf-8"))
+    alt.attach(email.mime.text.MIMEText(html_body, "html", "utf-8"))
+    msg.attach(alt)
 
     # Attach documents
     for doc_path in documents: