NFR-0003 — Protect catalog and amount integrity

Constraint: Runtime product identity and payment amounts shall derive only from the reviewed canonical catalog. Measurable target: Unknown identifiers and invalid quantities return validation errors; the create-order route ignores browser-supplied prices and reconstructs totals server-side. Scope: Catalog snapshot, PayPal BFF, and Features ESHOPV1-FT-1 and ESHOPV1-FT-3. Status: Proposed; implementation candidate. Verification: Catalog fixture tests, pricing tests, and PayPal order-construction tests. Evidence: Seven application tests and integrated CI task 1418 succeeded at 7fd6b88.