Add email verification for new account registration

[Mystery-CLI]

🔧 Title: Add email verification for new account registration

📘 Description
New users can register and immediately use the platform without verifying their email address. Unverified email addresses cannot receive notifications and reduce the platform's ability to recover accounts.

✅ Acceptance Criteria

• Send a verification email on registration with a time-limited token
• Add GET /api/auth/verify-email?token=... endpoint
• Block access to payment features until email is verified
• Add a "Resend verification email" button
• Add tests for the verification flow

🔧 Context: backend/src/routes/auth.js; backend/src/notifications/.

---