Canonical: all shop pages now assign og_url (reusing the existing og:url
assign), which the layout renders as <link rel="canonical">. Collection
pages strip the sort param so ?sort=price_asc doesn't create a duplicate
canonical.
robots.txt: dynamic controller disallows /admin/, /api/, /users/,
/webhooks/, /checkout/. Removed robots.txt from static_paths so it
goes through the router instead of Plug.Static.
sitemap.xml: auto-generated from all visible products + categories +
static pages, served as application/xml. 8 tests.
Also updates PROGRESS.md: marks tasks 55, 58, 59, 61, 62 as done.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
