add canonical URLs, robots.txt, and sitemap.xml

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>

lib/berrypod_web/components/layouts/shop_root.html.heex

@@ -28,6 +28,7 @@
     <meta property="og:description" content={og_description} />
     <meta property="og:type" content={assigns[:og_type] || "website"} />
     <%= if assigns[:og_url] do %>
+      <link rel="canonical" href={assigns[:og_url]} />
       <meta property="og:url" content={assigns[:og_url]} />
     <% end %>
     <%= if assigns[:og_image] do %>