Add List-Unsubscribe email headers for better deliverability and Gmail compliance

[kamsirichard]

Context

ScoutBot emails currently include an unsubscribe link in the footer (mailto: link). However, Gmail's bulk sender guidelines (effective Feb 2024) require one-click List-Unsubscribe headers for senders to more than 5,000 recipients per day.

Without these headers:

• Gmail may route ScoutBot emails to Promotions or Spam
• Bulk senders without headers face increased filtering from 2024 onwards

Task

Add standard List-Unsubscribe and List-Unsubscribe-Post headers to every outgoing digest email.

msg["List-Unsubscribe"]      = f"<mailto:{SENDER_EMAIL}?subject=Unsubscribe>"
msg["List-Unsubscribe-Post"] = "List-Unsubscribe=One-Click"

These go in _build_personal_email() in notify.py.

Stretch goal

Build a tiny unsubscribe endpoint so Gmail's one-click unsubscribe button works — clicking it in Gmail sends a POST request to a URL you control which removes the address from the sheet automatically. (Could be a minimal Flask app or a free serverless function.)

Files to touch

• notify.py — _build_personal_email()
• Optionally: new unsubscribe.py or hosted endpoint

References

• https://support.google.com/mail/answer/81126 (Gmail bulk sender guidelines)
• https://www.rfc-editor.org/rfc/rfc8058 (List-Unsubscribe-Post spec)