## Problem

The organization groups page showed each group's AI budget as the
group's per-member limit, so the total it displayed was effectively
group members × group budget. That ignores per-user budget overrides
charged to the group, so a group where one member has an override
reported a limit that doesn't match what its members can actually spend.

## Changes

- Add `total_spend_limit_micros` to the organization groups AI spend
payload, the combined budget of the members attributed to the group,
with each member's override replacing their share.
- Return `null` for the total when the group has no budget, since its
members spend without a cap.
- Both the organization groups and single group spend endpoints report
the new field, as they share the same query.
- Use the total as the denominator on the groups page AI budget column.

Depends on #27568

(cherry picked from commit 0b40950)