Flesh out dd0c/cost: ingestion with Welford optimistic locking, anomaly API, governance, baselines
- Ingestion API: batch cost events, Welford baseline update with optimistic locking (version column), anomaly detection inline
- Anomaly API: list (filtered), acknowledge, snooze (1-168h), mark expected, dashboard summary with hourly trend
- Governance API: mode status, promotion eligibility check with FP rate calculation
- Baseline API: list with computed stddev, reset per resource
- Data layer: withTenant() RLS wrapper, Zod config with ANOMALY_THRESHOLD
- Fastify server entry point