Build a cost ledger with budgets and alerts

The ledger stores what the API returned, never a cost. Every cost is computed when you query, from the price version valid on the event's date. A price correction, a missed rule or a new multiplier is then a code or table change followed by a re-run, not a migration. The rest builds on that: allocation is a GROUP BY with written rules, reconciliation turns pricing rules on one at a time and measures each, and budgets replay the month in time order. Everything runs on synthetic data, with no model calls.