Verify Recent Shipments Endpoint

Cron entry point: re-check recently-shipped experiments against production.

Triggered by EventBridge daily. For each vendor_config row written in the last lookback_days, runs the per-vendor verifier (if registered), auto-reverts the row + files a deployment_verification finding when production metrics have dropped past drop_threshold_pct.

Returns a per-row check list so the cron's emailed digest can render "what was inspected, what held, what got reverted."