feat(emails): add delivery, bounce, open, and click reporting

[mijinummi]

Pull Request: Add Email Delivery Reports

Overview

This PR implements issue #378 Add Email Delivery Reports.
It adds delivery analytics including delivery rate, bounce rate, open rate, and click rate.

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Changes Introduced

• Added EmailReport model for tracking events.
• Implemented EmailReportService for recording and aggregating metrics.
• Added /emails/reports endpoint to expose analytics.
• Integrated structured logging for email events.

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Acceptance Criteria ✅

• Delivery, bounce, open, click events tracked
• Metrics aggregated
• Reports endpoint available
• Logs include correlation IDs

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How to Test

1. Send test emails → simulate delivery, bounce, open, click events.
2. Call GET /emails/reports → verify metrics.
3. Check logs → events include correlationId and metadata.

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Contribution Notes

• Close Add Email Delivery Reports #378

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