[CardGrant] Add unclaimed grant reminder emails

### Describe the problem

I'd be great if hcb orgs had the ability to mass email ping users who have transactions without any receipts/grants that have been unclaimed for a while!

### Describe the feature/solution (optional)

_No response_

Start by locating the CardGrant implementation and the existing transaction, receipt, grant, and organization email flows. Clarify which unclaimed transactions qualify, how long they must remain unclaimed, who receives reminders, and how mass-email safeguards should work; done means the agreed reminder behavior is implemented and covered by tests.

Written by the indexing model from the issue text.