add page builder data model, cache, and block registry

Stage 1 of the page builder: Pages schema with 14 valid slugs,
BlockTypes registry (26 block types with settings schemas and data
loaders), Defaults module matching existing templates, ETS-backed
PageCache GenServer, and Pages context (get_page/save_page/reset_page
with cache -> DB -> defaults lookup). 34 tests, zero visual change.

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

priv/repo/migrations/20260226100000_create_pages.exs

@@ -0,0 +1,16 @@
+defmodule Berrypod.Repo.Migrations.CreatePages do
+  use Ecto.Migration
+
+  def change do
+    create table(:pages, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :slug, :string, null: false
+      add :title, :string, null: false
+      add :blocks, :map, default: "[]"
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:pages, [:slug])
+  end
+end