API: Add email notification service (welcome, audit complete, verification)

[Kami-no-san]

Description

Add email notification capabilities for key events: welcome emails, audit completion, blockchain verification confirmations.

Files to Modify

• apps/api/src/modules/notifications/notifications.service.ts — add email sending
• apps/api/src/modules/notifications/notifications.module.ts — register email provider

Files to Create

• apps/api/src/modules/notifications/email.service.ts — email sending logic
• apps/api/src/modules/notifications/templates/ — email templates

Acceptance Criteria

• Welcome email sent on user registration
• Audit completion notification with summary and link
• Blockchain verification confirmation email
• Uses a transactional email provider (Resend, SendGrid, or SES)
• Email provider configured via environment variables
• HTML and plain text versions of emails
• Email sending failures do not block API responses (fire-and-forget)
• Unit tests with mocked email provider