Reset Team Member Spend Fn

Reset a team member's tracked spend against their per-member budget.

A member's spend is tracked separately from both their own personal budget and the team's own budget (LiteLLM_TeamMembership.spend), so neither /user/update nor /team/update can clear it: this is the only endpoint that does. The cross-pod spend counter and cached membership reads are invalidated so the reset takes effect on the member's next request rather than waiting on the membership cache's TTL.