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>

test/berrypod_web/controllers/setup_controller_test.exs

@@ -0,0 +1,40 @@
+defmodule BerrypodWeb.SetupControllerTest do
+  use BerrypodWeb.ConnCase, async: true
+
+  import Berrypod.AccountsFixtures
+
+  alias Berrypod.Accounts
+
+  describe "GET /setup/login/:token" do
+    test "logs in with a valid token and redirects to /setup", %{conn: conn} do
+      user = user_fixture()
+      token = Accounts.generate_login_token(user)
+
+      conn = get(conn, ~p"/setup/login/#{token}")
+
+      assert redirected_to(conn) == ~p"/setup"
+      assert get_session(conn, :user_token)
+    end
+
+    test "redirects to /setup with error for invalid token", %{conn: conn} do
+      conn = get(conn, ~p"/setup/login/invalid-token")
+
+      assert redirected_to(conn) == ~p"/setup"
+      assert Phoenix.Flash.get(conn.assigns.flash, :error) =~ "Login failed"
+    end
+
+    test "token is consumed after use", %{conn: conn} do
+      user = user_fixture()
+      token = Accounts.generate_login_token(user)
+
+      # First use succeeds
+      conn1 = get(conn, ~p"/setup/login/#{token}")
+      assert redirected_to(conn1) == ~p"/setup"
+
+      # Second use fails (token consumed)
+      conn2 = get(conn, ~p"/setup/login/#{token}")
+      assert redirected_to(conn2) == ~p"/setup"
+      assert Phoenix.Flash.get(conn2.assigns.flash, :error) =~ "Login failed"
+    end
+  end
+end