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/router.ex

@@ -26,6 +26,11 @@ defmodule BerrypodWeb.Router do
     plug :put_secure_browser_headers
   end
 
+  # Minimal pipeline for robots.txt and sitemap.xml
+  pipeline :seo do
+    plug :put_secure_browser_headers
+  end
+
   pipeline :printify_webhook do
     plug BerrypodWeb.Plugs.VerifyPrintifyWebhook
   end
@@ -90,6 +95,14 @@ defmodule BerrypodWeb.Router do
     get "/health", HealthController, :show
   end
 
+  # SEO — crawlers need these without any session/auth overhead
+  scope "/", BerrypodWeb do
+    pipe_through [:seo]
+
+    get "/robots.txt", SeoController, :robots
+    get "/sitemap.xml", SeoController, :sitemap
+  end
+
   # Cart API (session persistence for LiveView)
   scope "/api", BerrypodWeb do
     pipe_through [:browser]