feat: prevent join requests with equal mail

lib/mv_web/emails/join_confirmation_email.ex

@@ -18,10 +18,16 @@ defmodule MvWeb.Emails.JoinConfirmationEmail do
   Uses the same SMTP configuration as the test mail (Settings or boot ENV) via
   `Mailer.deliver/2` with `Mailer.smtp_config/0` for consistency.
 
-  Called from the domain after a JoinRequest is created (submit flow).
+  Called from the domain after a JoinRequest is created (submit flow) or when
+  resending to an existing pending request.
+
+  ## Options
+    - `:resend` - If true, adds a short note that the link is being sent again for an existing request.
+
   Returns `{:ok, email}` on success, `{:error, reason}` on delivery failure.
   """
-  def send(email_address, token) when is_binary(email_address) and is_binary(token) do
+  def send(email_address, token, opts \\ [])
+      when is_binary(email_address) and is_binary(token) do
     confirm_url = url(~p"/confirm_join/#{token}")
     subject = gettext("Confirm your membership request")
 
@@ -29,7 +35,8 @@ defmodule MvWeb.Emails.JoinConfirmationEmail do
       confirm_url: confirm_url,
       subject: subject,
       app_name: Mailer.mail_from() |> elem(0),
-      locale: Gettext.get_locale(MvWeb.Gettext)
+      locale: Gettext.get_locale(MvWeb.Gettext),
+      resend: Keyword.get(opts, :resend, false)
     }
 
     email =