Add email notifications for certificate minting and retirement

[AnnabelJoe]

Description

Users have no notification when their certificates are minted or retired. Email notifications improve the user experience and provide a paper trail.

Acceptance Criteria

• Email sent when certificate is minted (includes certificate ID and kWh)
• Email sent when certificate is retired (includes beneficiary and tx hash)
• Email preferences configurable in user settings
• Emails use a professional template with SolarProof branding
• Unsubscribe link included

Priority

Low

Estimated Effort

Medium

Dependencies

Background job queue

Labels

enhancement, product, ux

[Anthony-19]

@Anthony-19 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi Maintainer, would love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Anthony-19 to this issue.