Allow integrations to thread replies to externally sent receipts

Allow integrations to connect replies to receipts sent through external email services with the correct existing Featurebase conversation.

This should work when the customer changes the subject or removes the visible case reference, using email threading metadata rather than guessing from the sender or subject.

The capability should also cover receipts already sent with a shared reply address.
Updating reply addresses for future receipts does not resolve those older emails.