Get the model spend of many runs at once

Return what each requested SignalScoutRun spent on model calls, summed from the $ai_generation events its sandbox produced. One query for the whole batch, cached per run: a settled run's total is final, a run still in progress reports what it has spent so far. available is false where the internal AI observability project holding those events can't be read, so an unknown cost never reads as zero. Staff-only — fleet spend is an internal operating number, and the events sit outside the project in the path. Strictly team-scoped — run ids belonging to another project contribute no rows.