Replacement for Firebase Send Email Extension

## Summary

Firebase has deprecated support for extensions - and is planning to decommission them on March 31, 2027. The only extension we currently use is the `firestore-send-email` extension - which manages the actual sending of rendered emails we write to our `emails` collection.

To keep our emails sending, we will need to migrate to a different email sending process. We don't have to address this now - Firebase claims they are going to release a migration plan for existing users in September 2026, so we can likely wait for that to decide on our approach here.

## Additional Links

Decommission notice: https://firebase.google.com/support/releases#extensions-deprecation