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/db/schema.sql

@@ -236,6 +236,29 @@ CREATE TABLE IF NOT EXISTS bot_config (
   CONSTRAINT chk_bot_config_singleton CHECK (id = 1)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
+-- Outbound email configuration (Gmail over OAuth2 / SMTP XOAUTH2). Singleton row
+-- (id = 1), mirroring bot_config: the DB only ever holds the AES-256-GCM-encrypted
+-- refresh token, never plaintext, and the client id/secret are NOT stored here —
+-- they are read live from the `google` auth_providers row. The refresh token is
+-- captured by the in-app "Connect Gmail" consent flow and is write-only over the
+-- admin API (never returned; responses expose only hasRefreshToken).
+CREATE TABLE IF NOT EXISTS email_config (
+  id                 INT PRIMARY KEY DEFAULT 1,
+  provider           VARCHAR(20)  NOT NULL DEFAULT 'gmail_oauth2',
+  enabled            TINYINT(1)   NOT NULL DEFAULT 0,
+  sender_email       VARCHAR(255) NULL,        -- connected Gmail address (from userinfo)
+  sender_name        VARCHAR(120) NULL,        -- optional From display name
+  refresh_token_enc  TEXT         NULL,        -- AES-256-GCM ciphertext, never exposed
+  status             VARCHAR(20)  NOT NULL DEFAULT 'unconfigured',
+  status_detail      VARCHAR(500) NULL,
+  last_verified_at   DATETIME     NULL,
+  updated_by         INT          NULL,
+  created_at         DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at         DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  CONSTRAINT fk_email_config_user FOREIGN KEY (updated_by) REFERENCES users(id) ON DELETE SET NULL,
+  CONSTRAINT chk_email_config_singleton CHECK (id = 1)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
 -- Discord bot moderation core (Phase 2). These tables are owned by the bot
 -- process (its own DB pool, bot/src/db.js) — the main server never reads or
 -- writes them. They live in the same physical database as everything else