Fix “Missing google_product_category” on Pinterest Catalog

Pinterest's google_product_category requirement is stricter than Google's. Where Google treats GPC as required only for select categories (apparel, software, weapons, media) and strongly recommended elsewhere, Pinterest treats it as required across all catalog items. Pinterest uses GPC directly for its own taxonomy mapping; without it, Pinterest can't categorise the item.

The migration trap: catalogs feeding both Google and Pinterest from one feed often have GPC populated on items where Google requires it but empty on items where Google doesn't. Pinterest then receives the feed and rejects items where GPC is missing, even though those same items work fine on Google. The fix: populate GPC universally across the catalog, not just for Google's required-categories list.

The fallback inference Google does (when GPC is missing, Google infers from title and other attributes) doesn't happen on Pinterest. Pinterest is stricter; missing means missing, not "we'll figure it out".

For high-quality categorisation, the deepest applicable taxonomy node is the rule that maximises both Google and Pinterest performance. Mapping to broad parent nodes works on Google but loses segmentation precision on Pinterest's shopping-pin distribution.

The Optimiser maps catalog products to Google's published taxonomy automatically, mapping to the deepest applicable node by analysing title, description, and image. Coverage typically hits 95%+ on first pass for retail catalogs.