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

server/src/server.js

@@ -8,7 +8,6 @@ const { ensureSchema, close } = require('./utils/db')
 const { seedDefaults, createInitialAdminFromEnv } = require('../db/seed')
 const settings = require('./model/settings/settings.model')
 const revokedSessions = require('./model/revokedSessions/revokedSessions.model')
-const mailer = require('./utils/mailer')
 const createLogger = require('./utils/logger')
 const { evaluateBotInternalKey } = require('./utils/botInternalKey')
 const pkg = require('../package.json')
@@ -30,7 +29,7 @@ async function start() {
     logFile: createLogger.logFilePath || 'disabled (console only)',
     db: `${process.env.DB_HOST || '127.0.0.1'}:${process.env.DB_PORT || 3306}/${process.env.DB_NAME || 'uomysticmoon'}`,
     cookieSecure: process.env.COOKIE_SECURE || 'auto',
-    smtp: mailer.isConfigured() ? 'configured' : 'not configured (mailto fallback)',
+    email: 'gmail-oauth2 (configured in admin → settings)',
   })
 
   // Fail fast if the server<->bot shared secret is weak/placeholder. Fatal in