IProductDataProvider abstraction per AGENTS.md §9; ProductService.
ResolveBarcodeAsync does local-first lookup with provider fallback
and persists accepted results. No real provider registered yet
(CW-STORY-04.4) — flow is fully tested via a fake provider.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

IProductService/ProductService return display-ready DTOs (BrandName/
ConceptName resolved via join) rather than raw Product entities.
ProductController routes match AGENTS.md §11; price/insight fields
deferred until CW-EPIC-05 exists.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>