Redact ticket title for non-public tickets in assignment notifications (#72)

sendAssignmentNotification() had the same missing-visibility gap as
#46 in a separate function: assigning a user to a confidential/internal
ticket broadcast the ticket title to the shared Matrix notify list
unconditionally when MATRIX_NOTIFY_ASSIGNMENTS is enabled.

Threaded visibility through using the same redactedTitle() helper
added for #46, wired up from the already-fetched ticket row in
assign_ticket.php. The assignee is still DMed directly regardless,
since being assigned gives them standing access to the ticket — but
because notify_users is one shared payload, they see the same redacted
title as everyone else on it rather than a personalized one. Verified
end-to-end with a local HTTP server capturing the webhook payload for
both public and confidential tickets.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015nCxwFFsy8ouMWzn56rPVP

helpers/NotificationHelper.php

@@ -280,8 +280,14 @@ class NotificationHelper
      * @param string|null $assigneeName    Display name of new assignee
      * @param string|null $assigneeMatrix  Matrix user ID of new assignee (to DM)
      * @param string|null $changedByDisplay
+     * @param string      $visibility      Ticket visibility; non-public titles are
+     *                                      redacted before being sent to the shared
+     *                                      notify list, same as sendTicketNotification().
+     *                                      The assignee is DMed directly regardless,
+     *                                      since they now have standing access to the
+     *                                      ticket by virtue of being assigned to it.
      */
-    public static function sendAssignmentNotification($ticketId, string $ticketTitle, ?string $assigneeName, ?string $assigneeMatrix, ?string $changedByDisplay = null): void
+    public static function sendAssignmentNotification($ticketId, string $ticketTitle, ?string $assigneeName, ?string $assigneeMatrix, ?string $changedByDisplay = null, string $visibility = 'public'): void
     {
         $notifyUsers = self::notifyUsers();
         // Also notify the assignee directly if we know their Matrix ID
@@ -295,7 +301,7 @@ class NotificationHelper
         self::fire([
             'event'        => 'assigned',
             'ticket_id'    => $ticketId,
-            'title'        => $ticketTitle,
+            'title'        => self::redactedTitle($ticketTitle, $visibility),
             'assignee'     => $assigneeName,
             'changed_by'   => $changedByDisplay,
             'url'          => UrlHelper::ticketUrl($ticketId),