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/components/layouts/admin.html.heex

@@ -126,6 +126,14 @@
                 <.icon name="hero-photo" class="size-5" /> Media
               </.link>
             </li>
+            <li>
+              <.link
+                navigate={~p"/admin/newsletter"}
+                class={admin_nav_active?(@current_path, "/admin/newsletter")}
+              >
+                <.icon name="hero-megaphone" class="size-5" /> Newsletter
+              </.link>
+            </li>
             <li>
               <.link
                 href={~p"/admin/theme"}