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/controllers/seo_controller.ex

@@ -0,0 +1,73 @@
+defmodule BerrypodWeb.SeoController do
+  use BerrypodWeb, :controller
+
+  alias Berrypod.Products
+
+  def robots(conn, _params) do
+    base = BerrypodWeb.Endpoint.url()
+
+    content = """
+    User-agent: *
+    Allow: /
+    Disallow: /admin/
+    Disallow: /api/
+    Disallow: /users/
+    Disallow: /webhooks/
+    Disallow: /checkout/
+
+    Sitemap: #{base}/sitemap.xml
+    """
+
+    conn
+    |> put_resp_content_type("text/plain")
+    |> send_resp(200, content)
+  end
+
+  def sitemap(conn, _params) do
+    base = BerrypodWeb.Endpoint.url()
+    products = Products.list_visible_products()
+    categories = Products.list_categories()
+
+    static_pages = [
+      {"/", "daily", "1.0"},
+      {"/collections/all", "daily", "0.9"},
+      {"/about", "monthly", "0.5"},
+      {"/contact", "monthly", "0.5"},
+      {"/delivery", "monthly", "0.5"},
+      {"/privacy", "monthly", "0.3"},
+      {"/terms", "monthly", "0.3"}
+    ]
+
+    category_pages =
+      Enum.map(categories, fn cat ->
+        {"/collections/#{cat.slug}", "daily", "0.8"}
+      end)
+
+    product_pages =
+      Enum.map(products, fn product ->
+        {"/products/#{product.slug}", "weekly", "0.9"}
+      end)
+
+    all_pages = static_pages ++ category_pages ++ product_pages
+
+    entries =
+      Enum.map_join(all_pages, "\n", fn {path, changefreq, priority} ->
+        "  <url>\n" <>
+          "    <loc>#{base}#{path}</loc>\n" <>
+          "    <changefreq>#{changefreq}</changefreq>\n" <>
+          "    <priority>#{priority}</priority>\n" <>
+          "  </url>"
+      end)
+
+    xml = """
+    <?xml version="1.0" encoding="UTF-8"?>
+    <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+    #{entries}
+    </urlset>
+    """
+
+    conn
+    |> put_resp_content_type("application/xml")
+    |> send_resp(200, xml)
+  end
+end