chore(core): decouple email delivery from ActionMailer

[ArtieReus]

Summary

This PR removes ActionMailer’s built-in delivery pipeline (deliver_now / deliver_later) from inquiry emails and switches fully to our custom HTTP-based email client.

-ActionMailer is now used only for template rendering, while actual delivery is handled explicitly via send_custom_email.

Changes Made

• Removed all calls to deliver_now / deliver_later.
• Mailer methods now execute synchronously and send emails via the custom HTTP client, no need of mail jobs anymore.
• Fixes ineffective error handling: rescuing Net::SMTPError around deliver_later was misleading, since delivery happens asynchronously in a background job and exceptions would never be raised at the call site.
• Introduced EmailDeliveryError to standardize email delivery failures
• Updated error handling to rescue meaningful, synchronous delivery errors

Related Issues

• [Task](core-email): clean up old cronus implementation and remove related secrets #1873

Checklist

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have made corresponding changes to the documentation (if applicable).
• My changes generate no new warnings or errors.