Sign API returns presigned upload URL + storage key.
Dev: files served through auth-gated /api/files/dev route with path-traversal protection.
Prod: R2 presigned URLs for upload and time-limited download.
7 event templates: po-submitted, po-approved, po-rejected, edits-requested,
vendor-id-needed, payment-processed, receipt-confirmed.
Notifier uses Resend in production and console.log in development.
10 statuses, 11 transitions. Each transition declares allowedRoles,
requiresNote flag and sideEffects (which email groups to notify).
Helpers: getTransition, canPerformAction, getAvailableActions, requiresNote.