feat: add email unsubscribe management

[Nimatstar]

Summary

Implements email unsubscribe management for compliance, including one-click unsubscribe, preference centre, and resubscribe mechanism.

Changes

• src/email-unsubscribe/entities/unsubscribe-token.entity.ts - Token entity with expiry and single-use enforcement
• src/email-unsubscribe/dto/unsubscribe.dto.ts - DTOs for unsubscribe, resubscribe, and preference update
• src/email-unsubscribe/email-unsubscribe.service.ts - Service: token generation, one-click unsubscribe, resubscribe, preference management, status lookup
• src/email-unsubscribe/email-unsubscribe.controller.ts - REST endpoints: POST /email-unsubscribe/unsubscribe, POST /email-unsubscribe/resubscribe, POST /email-unsubscribe/preferences, GET /email-unsubscribe/status/:email
• src/email-unsubscribe/email-unsubscribe.module.ts - Module wiring

Acceptance Criteria

• Unsubscribe link in all emails (token generation via generateUnsubscribeToken)
• One-click unsubscribe (POST /email-unsubscribe/unsubscribe)
• Preference center for email types (POST /email-unsubscribe/preferences)
• Resubscribe mechanism (POST /email-unsubscribe/resubscribe)

closes #589

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