feat: robust email sending updates

[amandazhuyilan]

Description

AAI-538: implement a tighter SES retry policy for queued emails

Changes

• Added first_attempt_at tracking plus a schema migration so we can limit delivery attempts per notification.
• Updated the scheduler to cap emails at 2 attempts, retry only on transient/throttling/network failures, randomly delay retries 15–30 minutes, and stop once an hour has passed since the first try.
  • Max attempts per email: 2 (initial + 1 retry)
  • Retry only on temporary errors: SES transient / throttling / network
  • Retry delay: 15–30 minutes after the first failure
  • Max window: 1 hour from first attempt (no retries after that)
• Updated tests

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.

[marius-mather]

Looks good, but we might need to allow for more retries in future, so last_attempt_at might make more sense than first_attempt_at

[marius-mather]

good to go