Manually notify volunteers for a class

Context

When a class is assigned/scheduled to volunteers, we want to notify them by email.
We already have infrastructure for background jobs (pgBoss) and SMTP email delivery.

Issue

There is currently no manual workflow for notifying volunteers when a class is scheduled/assigned to them.
We need an ad-hoc notification job that can be triggered for a class assignment and sends email notifications to the relevant volunteers.

Notes

This should use our existing pgBoss setup for queuing/scheduling and existing SMTP service for sending email. jobService.ts
Include safeguards to prevent duplicate notifications for the same class assignment action.
Keep this scoped to backend/email job behavior (not a full notification preferences feature).