auto-confirm admin during setup, skip email verification

Setup wizard no longer requires email delivery. Admin account is
auto-confirmed and auto-logged-in via token redirect. Adds setup
secret gate for prod (logged on boot), SMTP env var config in
runtime.exs, email_configured? helper, and admin warning banner
when email isn't set up. Includes plan files for this task and
the follow-up email settings UI.

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -18,6 +18,15 @@
       </.link>
     </header>
 
+    <%!-- email warning banner --%>
+    <div :if={!@email_configured} class="admin-banner-warning">
+      <.icon name="hero-exclamation-triangle" class="size-5 shrink-0" />
+      <p>
+        Email delivery not configured. Customers won't receive order confirmations.
+        Set <code>SMTP_HOST</code> to enable email.
+      </p>
+    </div>
+
     <%!-- page content --%>
     <main class="flex-1 p-4 sm:p-6 lg:p-8">
       <div class="mx-auto max-w-5xl">