Resolve a bare product slug to its canonical org-scoped home

Same semantics as /v1/lookups/source-by-slug but for products. Returns { productId, productSlug, orgSlug } so callers can rewrite to /v1/orgs/:orgSlug/products/:productSlug before the bare path stops resolving slugs. Oldest match by (createdAt, id). Carries Sunset: Sun, 01 Nov 2026 00:00:00 GMT on success.