Modernize email: Gmail OAuth2 sending, configured under Settings

Retire env-var SMTP basic-auth and send the contact form through Gmail over
OAuth2 (SMTP XOAUTH2), configured in Admin -> Settings -> Email via an in-app
"Connect Gmail" consent flow. Reuses the existing google SSO OAuth client; the
captured refresh token is stored AES-GCM-encrypted (write-only over the API,
never returned), mirroring the auth-provider and Discord-bot secret patterns.

- schema: new email_config singleton table (mirrors bot_config)
- model: emailConfig.{db,model} with encrypted refresh token + getSafe/getWithSecret
- mailer: nodemailer OAuth2 transport (client id/secret from the google provider
  row), contact recipient = contact_email setting, mailto: fallback preserved,
  plus sendTest()
- routes/controller: /admin/email config, connect start+callback (ssoState CSRF
  + PKCE), test, disconnect
- client: EmailDelivery section on the Settings page + api methods; Settings copy
  now spells out that contact_email is the delivery recipient
- docs/env: drop SMTP_*/CONTACT_TO from env examples; update README/BACKEND_DESIGN
- tests: emailConfig.model + mailer suites (8 new; full suite 142 pass)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XKeCQEJZr1AFJN4Bgcmvh3

website/BACKEND_DESIGN.md

@@ -234,10 +234,13 @@ who"; `activity_log` provides the history feed.
 
 ## 7. Email
 
-`utils/mailer.js` (nodemailer) configured from `SMTP_HOST/PORT/USER/PASS`, sending to
-`CONTACT_TO` (default UOMysticmoon@gmail.com). No Gmail password in code — env only.
-If SMTP is unconfigured, `POST /public/contact` returns `{fallback:"mailto", email}` so the
-client renders a `mailto:` link instead. Site mode changes / errors never leak SMTP creds.
+`utils/mailer.js` (nodemailer) sends through **Gmail over OAuth2 (SMTP XOAUTH2)**, configured in
+Admin → Settings → Email — not env. The mailbox is authorized by an in-app "Connect Gmail" consent
+flow (`/admin/email/*`) that captures a refresh token, stored AES-GCM-encrypted in the `email_config`
+singleton (never returned over the API). The OAuth client id/secret are reused from the `google`
+auth-providers row. Recipient is the `contact_email` site setting. If email is unconfigured/disabled,
+`POST /public/contact` returns `{fallback:"mailto", email}` so the client renders a `mailto:` link
+instead. Errors never leak credentials.
 
 ---
 
@@ -287,11 +290,7 @@ COOKIE_SECURE=true
 COOKIE_NAME=uomm_token
 ADMIN_USERNAME=
 ADMIN_PASSWORD=
-SMTP_HOST=
-SMTP_PORT=587
-SMTP_USER=
-SMTP_PASS=
-CONTACT_TO=UOMysticmoon@gmail.com
+# Email: configured in Admin → Settings → Email (Gmail OAuth2), not via env
 CLIENT_ORIGIN=http://localhost:5173
 ```