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/router.ex

@@ -146,6 +146,9 @@ defmodule BerrypodWeb.Router do
   scope "/", BerrypodWeb do
     pipe_through [:browser]
 
+    # Token-based auto-login after setup account creation
+    get "/setup/login/:token", SetupController, :login
+
     live_session :setup,
       on_mount: [{BerrypodWeb.UserAuth, :mount_current_scope}] do
       live "/setup", Setup.Onboarding, :index