feat(backend): Implement email notification service with templates

[KuchiMercy]

Description

nodemailer installed but no email service exists. Build email service with HTML templates, preference management, and async delivery.

Config: SMTP_HOST/PORT/USER/PASS, EMAIL_FROM in .env.example

Acceptance Criteria

• EmailService in src/notifications/senders/
• HTML templates: Welcome, Funded, Milestone Completed/Approved, Dispute Raised/Resolved, Refunded, Deadline Approaching
• Each template: branding header, event details, CTA button, unsubscribe footer
• String interpolation or handlebars templating
• Integrate with NotificationPreferenceService (opt-out)
• Queue emails for async delivery
• SMTP failure retry: 3x exponential backoff
• Log all sends: recipient, template, status
• Dry-run mode for development
• EMAIL_ENABLED flag (default false)
• Unit tests for template rendering
• Integration test with mock SMTP

Points: 150

[deedee-code]

@deedee-code has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, please i will like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @deedee-code to this issue.