add url_slug column to pages table

Adds url_slug field for custom page URLs:
- Migration with unique partial index on non-null slugs
- Page schema with validation (format, uniqueness, reserved words)
- effective_url/1 helper to resolve custom or default slug
- Default page configs updated with url_slug field

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

priv/repo/migrations/20260329210409_add_url_slug_to_pages.exs

@@ -0,0 +1,12 @@
+defmodule Berrypod.Repo.Migrations.AddUrlSlugToPages do
+  use Ecto.Migration
+
+  def change do
+    alter table(:pages) do
+      add :url_slug, :string
+    end
+
+    # Unique index but only for non-null values
+    create unique_index(:pages, [:url_slug], where: "url_slug IS NOT NULL")
+  end
+end