Track and filter credits on support tickets

Improved

Two changes make credits on support tickets easier to follow.

The support-requests table now shows the credited amount on any ticket, whatever its reason.

A new Credit filter sorts the table by credit status: No credit for tickets closed without one, Credit on next invoice for a credit approved but not yet applied, and Credited for those already settled, with the amount shown. The filter covers five ticket reasons for now, with the others to follow.

To use it: apply the Credit filter on the support-requests table. Credited amounts are tracked automatically.