add custom page data model with split changesets and CRUD context

Stage 1 of custom CMS pages. Adds type/published/meta/nav fields to
pages schema, splits changeset into system vs custom (with slug format
validation and reserved path exclusion), adds create/update/delete
functions with auto-redirect on slug change, and warms custom pages
in ETS cache. 62 pages tests, 1426 total.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

priv/repo/migrations/20260228014939_add_custom_page_fields.exs

@@ -0,0 +1,14 @@
+defmodule Berrypod.Repo.Migrations.AddCustomPageFields do
+  use Ecto.Migration
+
+  def change do
+    alter table(:pages) do
+      add :type, :string, default: "system"
+      add :published, :boolean, default: true
+      add :meta_description, :string
+      add :show_in_nav, :boolean, default: false
+      add :nav_label, :string
+      add :nav_position, :integer
+    end
+  end
+end