Post Training Cost

Record a training run's realized provider cost (COGS) for estimator accuracy.

Machine-to-machine: the flash control plane's reconciliation job calls this after a run finishes, authenticated with the operator internal key (NOT a user key -- this is COGS, not a customer charge). Upserts training_run_cost keyed by runId; the org and the charged estimate are joined in later (the training_cost_accuracy view), so no org id is needed here. Idempotent: a re-pulled run (provider billing lags) records once.