fix: use wallet OTP email for email changes

[jmgasper]

Summary

• Replace the activation-email template used by email-change ownership checks with the same SendGrid OTP template used by wallet withdrawals.
• Send the wallet template contract: member name in name and the six-digit code in otp.
• Use the member first and last name with handle fallback, plus the same v3 email event shape as tc-finance-api.
• Add the shared template setting to the sample environment and documentation.

Root cause

The email-change flow used SENDGRID_RESEND_ACTIVATION_EMAIL_TEMPLATE_ID with handle/code fields. That selects the registration activation email and leaves the wallet template name/otp fields empty.

Testing

• pnpm lint
• pnpm build
• pnpm exec jest src/api/user/email-change.service.spec.ts --runInBand (4 tests)

The regression test asserts the complete outbound payload, including the wallet template ID d-2d0ab9f6c9cc4efba50080668a9c35c1, name, otp, sender, recipient, and v3 version.