feat: email verification via Resend + admin user management

- Add email verification flow: register sends verify link, login blocks unverified accounts, 24h token expiry, resend endpoint (3/h rate limit).
- Add mailer module using Resend with branded HTML + plaintext template.
- Extend admin dashboard: verified-status column, toggle verify/unverify buttons, promote/demote admin role, delete any non-self user.
- Migrate users table: email_verified, verify_token, verify_expires columns.
- Load env via dotenv; add .env to gitignore.

.gitignore

@@ -2,3 +2,4 @@ node_modules/
 data.db
 storage/
 CLAUDE.md
+.env