Issue #65 — Transactional email service

[GoSTEAN]

Title: [Notifications] Implement transactional email service

Tags: backend, notifications, feature

Description:
Send transactional emails (verification, password reset, market resolved, winnings available) via SMTP/SendGrid.

Acceptance Criteria:

• email.service.ts exposes sendEmail(to, template, data).
• Templates: verify_email, reset_password, market_resolved, winnings_available, dispute_filed, dispute_resolved.
• HTML templates in src/email/templates/.
• Provider configured via EMAIL_PROVIDER env (smtp or sendgrid).
• Fails gracefully — email errors are logged but do not throw to caller.
• Unit test: mock transport; verify correct template and recipient used.

Files: src/services/email.service.ts, src/email/templates/

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