Implement Nodemailer Email Service for Welcome, Verification Complete, and Risk Alert Notifications

[mftee]

Description:
Implement a standalone mail service using Nodemailer that can send transactional emails (welcome, verification complete, risk alert).

Tasks:

• Create src/mail/mail.module.ts and mail.service.ts
• Configure a Nodemailer transport using MAIL_HOST, MAIL_PORT, MAIL_USER, MAIL_PASSWORD, MAIL_FROM from ConfigService
• Implement sendWelcome(to: string, name: string): Promise<void>
• Implement sendVerificationComplete(to: string, documentTitle: string, txHash: string): Promise<void>
• Implement sendRiskAlert(to: string, documentTitle: string, flags: string[]): Promise<void>
• Use simple inline HTML templates (no external templating engine required)

Acceptance Criteria:

• All three email methods send without error when SMTP credentials are valid
• Missing SMTP config causes the app to log a warning but not crash (graceful degradation)
• Emails contain the relevant data passed in the method arguments