docs(email): SMTP setup, the three postures, and the upgrade note

The operator-facing half of engagement Phase 1. README's stack table and
security section, plus both .env.example files, all pointed at the
removed Connect Gmail flow.

The env comments now name the three supported postures rather than one
provider — a relay as the recommendation, smtp.gmail.com:587 with an app
password as the shortest migration, an unauthenticated local MTA as the
third — and point at docs/website/UPGRADE_NOTES.md for the deployment
this actually happens to.

The OpenAPI spec is regenerated: two routes gone, three annotations
rewritten, and the dashboard's new warnings[] documented.

Co-Authored-By: Claude <noreply@anthropic.com>

server/.env.example

@@ -80,10 +80,12 @@ TOTP_CHALLENGE_TTL=5m
 ADMIN_USERNAME=admin
 ADMIN_PASSWORD=change-me-admin-password
 
-# Email is configured in Admin → Settings → Email (Gmail over OAuth2), not here.
-# It reuses the Google auth provider's OAuth client and stores an encrypted
-# refresh token in the DB. The contact recipient is the `contact_email` site
+# Email is configured in Admin → Settings → Email, not here: pick a mail
+# transport (SMTP) and enter its host, port and credentials, stored encrypted in
+# the DB. A relay is the recommended posture; smtp.gmail.com:587 with an app
+# password is the simplest. The contact recipient is the `contact_email` site
 # setting; while email is unconfigured the contact form falls back to a mailto: link.
+# Upgrading from the removed Gmail connect flow: see docs/website/UPGRADE_NOTES.md.
 
 CLIENT_ORIGIN=http://localhost:5173