feat: view email sender in admin dashboard

[amandazhuyilan]

Description

Improve email notification debugging capabilities via adding sender information in db.

Changes

• Introduced email_settings.py so a single DEFAULT_EMAIL_SENDER (overridable via env, currently amanda@biocommons.org.au) is shared by the backend.
• a new from_address added in EmailNotification, with the field indexed and defaulting to the shared sender so existing code keeps working.
• services/email_queue.py accepts an optional from_address when queuing mail and falls back to the shared default.
• 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.

How to Test Manually (if necessary)

<Describe how reviewers can manually, if necessary, test the changes>

[marius-mather]

A few small things to look at here - particularly the way we set the config. Would be nice if we could stick to using pydantic-settings and .env for consistentcy

[marius-mather]

Looks good to me