add page url slug management functions

Context functions for custom page URLs:
- list_pages_with_custom_urls/0 for admin overview
- get_published_page_by_effective_url/1 for routing lookups
- update_page_url_slug/2 with automatic redirect creation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/berrypod/pages.ex

@@ -90,6 +90,28 @@ defmodule Berrypod.Pages do
     list_system_pages() ++ list_custom_pages()
   end
 
+  @doc "Lists pages that have a custom URL slug set. Used by Routes cache."
+  def list_pages_with_custom_urls do
+    from(p in Page, where: not is_nil(p.url_slug))
+    |> Repo.all()
+  end
+
+  @doc "Gets a published page by its effective URL (url_slug or slug)."
+  def get_published_page_by_effective_url(url) do
+    # First check for custom url_slug match
+    case Repo.one(from p in Page, where: p.url_slug == ^url and p.published == true) do
+      nil ->
+        # Fall back to slug match for custom pages
+        Repo.one(
+          from p in Page,
+            where: p.slug == ^url and p.type == "custom" and p.published == true
+        )
+
+      page ->
+        page
+    end
+  end
+
   # ── Write ─────────────────────────────────────────────────────────
 
   @doc """
@@ -272,7 +294,91 @@ defmodule Berrypod.Pages do
       meta_description: page.meta_description,
       show_in_nav: page.show_in_nav,
       nav_label: page.nav_label,
-      nav_position: page.nav_position
+      nav_position: page.nav_position,
+      url_slug: page.url_slug
     }
   end
+
+  # ── URL customisation ────────────────────────────────────────────
+
+  @doc """
+  Updates a page's custom URL slug. Creates a redirect from the old
+  effective URL to the new one, and invalidates the R module cache.
+
+  Pass nil or empty string to clear the custom URL.
+  """
+  def update_page_url_slug(%Page{} = page, url_slug) do
+    old_effective_url = Page.effective_url(page)
+
+    # Normalise empty string to nil
+    url_slug = if url_slug == "", do: nil, else: url_slug
+
+    changeset_fn =
+      if Page.system_slug?(page.slug),
+        do: &Page.system_changeset/2,
+        else: &Page.custom_changeset/2
+
+    result =
+      page
+      |> changeset_fn.(%{url_slug: url_slug})
+      |> Repo.update()
+
+    case result do
+      {:ok, updated} ->
+        new_effective_url = Page.effective_url(updated)
+
+        # Create redirect if URL changed
+        if old_effective_url != new_effective_url do
+          # Delete any stale redirect FROM the new URL (it's now a live page)
+          delete_stale_redirect("/#{new_effective_url}")
+
+          # Create redirect from old URL to new URL
+          Berrypod.Redirects.create_auto(%{
+            from_path: "/#{old_effective_url}",
+            to_path: "/#{new_effective_url}",
+            source: "auto_slug_change"
+          })
+        end
+
+        # Invalidate caches synchronously to avoid race conditions
+        PageCache.invalidate(page.slug)
+        BerrypodWeb.R.invalidate_all_sync()
+
+        {:ok, updated}
+
+      error ->
+        error
+    end
+  end
+
+  def update_page_url_slug(slug, url_slug) when is_binary(slug) do
+    case get_page_struct(slug) do
+      nil ->
+        # Page doesn't exist in DB - create it from defaults if it's a system page
+        if Page.system_slug?(slug) do
+          defaults = Defaults.for_slug(slug)
+
+          case save_page(slug, defaults) do
+            {:ok, page} -> update_page_url_slug(page, url_slug)
+            error -> error
+          end
+        else
+          {:error, :not_found}
+        end
+
+      page ->
+        update_page_url_slug(page, url_slug)
+    end
+  end
+
+  # When a URL becomes a live page, delete any redirect from that URL
+  defp delete_stale_redirect(path) do
+    case Repo.one(from r in Berrypod.Redirects.Redirect, where: r.from_path == ^path) do
+      %Berrypod.Redirects.Redirect{} = redirect ->
+        Berrypod.Redirects.delete_redirect(redirect)
+
+      nil ->
+        :ok
+    end
+  end
 end