feat: prevent join requests with equal mail

lib/mv_web/emails/join_already_pending_email.ex

@@ -0,0 +1,43 @@
+defmodule MvWeb.Emails.JoinAlreadyPendingEmail do
+  @moduledoc """
+  Sends an email when someone submits the join form with an address that already
+  has a submitted (confirmed) application under review.
+
+  Used for anti-enumeration: the UI shows the same success message; only the email
+  informs the recipient. Uses the unified email layout.
+  """
+  use Phoenix.Swoosh,
+    view: MvWeb.EmailsView,
+    layout: {MvWeb.EmailLayoutView, "layout.html"}
+
+  use MvWeb, :verified_routes
+  import Swoosh.Email
+  use Gettext, backend: MvWeb.Gettext, otp_app: :mv
+
+  alias Mv.Mailer
+
+  @doc """
+  Sends the "application already under review" notice to the given address.
+
+  Returns `{:ok, email}` on success, `{:error, reason}` on delivery failure.
+  """
+  def send(email_address) when is_binary(email_address) do
+    subject = gettext("Membership application – already under review")
+
+    assigns = %{
+      subject: subject,
+      app_name: Mailer.mail_from() |> elem(0),
+      locale: Gettext.get_locale(MvWeb.Gettext)
+    }
+
+    email =
+      new()
+      |> from(Mailer.mail_from())
+      |> to(email_address)
+      |> subject(subject)
+      |> put_view(MvWeb.EmailsView)
+      |> render_body("join_already_pending.html", assigns)
+
+    Mailer.deliver(email, Mailer.smtp_config())
+  end
+end