feat: add join confirmation and mail templating

lib/mv_web/emails/join_confirmation_email.ex

@@ -0,0 +1,33 @@
+defmodule MvWeb.Emails.JoinConfirmationEmail do
+  @moduledoc """
+  Sends the join request confirmation email (double opt-in) using the unified email layout.
+  """
+  use Phoenix.Swoosh,
+    view: MvWeb.EmailsView,
+    layout: {MvWeb.EmailLayoutView, :layout}
+
+  use MvWeb, :verified_routes
+  import Swoosh.Email
+  use Gettext, backend: MvWeb.Gettext, otp_app: :mv
+
+  alias Mv.Mailer
+
+  @doc """
+  Sends the join confirmation email to the given address with the confirmation link.
+
+  Called from the domain after a JoinRequest is created (submit flow).
+  """
+  def send(email_address, token) when is_binary(email_address) and is_binary(token) do
+    confirm_url = url(~p"/confirm_join/#{token}")
+    subject = gettext("Confirm your membership request")
+
+    new()
+    |> from(Mailer.mail_from())
+    |> to(email_address)
+    |> subject(subject)
+    |> put_view(MvWeb.EmailsView)
+    |> put_layout({MvWeb.EmailLayoutView, "layout.html"})
+    |> render_body("join_confirmation.html", %{confirm_url: confirm_url, subject: subject})
+    |> Mailer.deliver!()
+  end
+end