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

.env.example

@@ -53,13 +53,10 @@ TOTP_CHALLENGE_TTL=5m
 ADMIN_USERNAME=
 ADMIN_PASSWORD=
 
-# Email (optional). If SMTP_HOST is blank, the contact endpoint tells the
-# client to fall back to a mailto: link instead.
-SMTP_HOST=
-SMTP_PORT=587
-SMTP_USER=
-SMTP_PASS=
-CONTACT_TO=UOMysticmoon@gmail.com
+# Email is configured in Admin → Settings → Email (Gmail over OAuth2), not via
+# env. It reuses the Google auth provider's OAuth client and stores an encrypted
+# refresh token in the DB. Until it's connected, the contact form falls back to
+# a mailto: link (recipient = the `contact_email` site setting).
 
 # CORS — only needed for local dev when the Vite dev server is a different origin.
 CLIENT_ORIGIN=http://localhost:5173