add newsletter and email campaigns

Subscribers with double opt-in confirmation, campaign composer with
draft/scheduled/sent lifecycle, admin dashboard with overview stats,
CSV export, and shop signup form wired into page builder blocks.

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

lib/berrypod_web/controllers/newsletter_export_controller.ex

@@ -0,0 +1,16 @@
+defmodule BerrypodWeb.NewsletterExportController do
+  use BerrypodWeb, :controller
+
+  alias Berrypod.Newsletter
+
+  def export(conn, _params) do
+    csv = Newsletter.export_all_subscribers_csv()
+    today = Date.to_iso8601(Date.utc_today())
+    filename = "newsletter-subscribers-#{today}.csv"
+
+    conn
+    |> put_resp_content_type("text/csv")
+    |> put_resp_header("content-disposition", ~s(attachment; filename="#{filename}"))
+    |> send_resp(200, csv)
+  end
+end