Product Feed V2

Stream the gzipped XML product feed from the merchant's own domain.

Backs the new product_feeds system in extension-backend. The token is stable per-feed (rotated on regen via S3 key reuse), and the underlying object is uploaded with Content-Encoding: gzip, so we pass the raw bytes through unchanged and mirror the upstream headers so crawlers (Meta Catalog, TikTok, Google Merchant Center) decompress correctly.

HEAD is forwarded upstream as HEAD: crawlers probe it before fetching, and we answer with the object's headers without streaming the body.