feat(webserver): Implement send_mail method in EmailService (#1233)

* Create send_mail method for EmailService

* Remove todo

* Use mutable Vec reference so unsent emails remain in memory for retrying

* Document mail sending behavior

* Remember to set credentials

* [autofix.ci] apply automated fixes

* [autofix.ci] apply automated fixes (attempt 2/3)

* Update README.md (#1234)

* Create mail service in server context

* Apply suggestions

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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Meng Zhang <[email protected]>