## Goal Prevent negative balances under concurrent usage and estimate tokens when upstream usage metadata is unavailable. ## Acceptance Criteria - [ ] Wallet row is locked or otherwise protected during charge transaction. - [ ] Concurrent requests cannot push balance below zero. - [ ] Token estimation fallback is deterministic and marked estimated. - [ ] Tests cover insufficient credit and concurrent charge behavior.
Goal
Prevent negative balances under concurrent usage and estimate tokens when upstream usage metadata is unavailable.
Acceptance Criteria