[Enhancement]: Alert users when prepaid credits balance is going to run out of funds

### Affected Area

None

### Problem Statement

Currently, users are not alerted when their prepaid credit balance is running low.

An alert should be triggered when the prepaid credit balance is going to run out of funds.

### Proposed Solution

_No response_

Start by locating the prepaid-credit balance handling and any existing user alert or notification entry points. Clarify the low-balance threshold, alert channel, timing, and repeat behavior with the maintainers; done means users are alerted before prepaid funds run out and the behavior is covered by tests.

Written by the indexing model from the issue text.