Post Training Usage Precheck

Verify-only budget pre-flight before a managed flash run is allocated a GPU.

Machine-to-machine: the flash control plane calls this at submit (internal key + explicit org id) with the run's flash.cost estimate. Returns 402 when the org can't cover the estimate (no balance and no card+auto-topup), so the control plane rejects the run BEFORE any GPU is allocated -- instead of letting it train to completion and only then failing to bill. This moves no money: the real debit still happens at completion from the actual cost_usd.