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/controllers/setup_controller.ex

@@ -0,0 +1,37 @@
+defmodule BerrypodWeb.SetupController do
+  use BerrypodWeb, :controller
+
+  alias Berrypod.Accounts
+  alias BerrypodWeb.UserAuth
+
+  @doc """
+  Logs in a user via a setup login token.
+
+  The setup wizard generates a token after creating the admin account,
+  then redirects here to set the session cookie (LiveViews can't do that).
+  """
+  def login(conn, %{"token" => token}) do
+    # Validate token first — login_user_by_magic_link crashes on invalid base64
+    if Accounts.get_user_by_magic_link_token(token) do
+      case Accounts.login_user_by_magic_link(token) do
+        {:ok, {user, tokens_to_disconnect}} ->
+          UserAuth.disconnect_sessions(tokens_to_disconnect)
+
+          conn
+          |> put_session(:user_return_to, ~p"/setup")
+          |> UserAuth.log_in_user(user)
+
+        _ ->
+          login_failed(conn)
+      end
+    else
+      login_failed(conn)
+    end
+  end
+
+  defp login_failed(conn) do
+    conn
+    |> put_flash(:error, "Login failed — please try again.")
+    |> redirect(to: ~p"/setup")
+  end
+end