Implement email notification service`

[Nemenwq]

Dependencies: Issue #26. Trigger points depend on which events exist (e.g. Issue #34).

Context:
Merchants receive email on key events.

Implementation Notes:

• Start with a provider-agnostic interface (SMTP, Sendgrid, etc. can be plugged in later).
• For local dev, use a mail catcher (optional) or log emails to console.

Scope:

• In scope: Service + templates + queue.
• Out of scope: Full marketing email system.

Files: backend/notification_service/src/email/