fix: remove send_email flag and set up email queue in DB

[amandazhuyilan]

Description

AAI-461: Remove unused send_email flag and set up email queue in DB

Changes

• removes the Settings.send_email flag so there’s no configuration gate keeping emails from being dispatched (we will do this through AWS IAM)
• Removed previously added EmailService background task
• Added a durable email queue:
  • introduced EmailStatusEnum, the EmailNotification model with queue helpers
  • a scheduler task and job wiring. Pending emails are persisted, retried with backoff, and emitted periodically by APScheduler.
  • Email resend in 300 seconds if unsuccessful
• updated tests to validate queued emails and the new worker

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)
• For any new secrets, I have updated the shared spreadsheet and the GitHub Secrets.

How to Test Manually (if necessary)

All updated and existing unit tests shall pass in CI!

[marius-mather]

This looks fine for what it is (removing the config flag). Not sure if we want to work on a more robust email send service now, or leave that for later.

[marius-mather]

This all looks pretty good to me - I've flagged a few things you could consider changing but otherwise approved.

[marius-mather]

looks good to me!