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

api/assign_ticket.php

@@ -76,7 +76,8 @@ if ($assignedTo === null || $assignedTo === '') {
                 $ticket['title'] ?? "Ticket #{$ticketId}",
                 $assigneeName,
                 $assigneeMatrix,
-                $changedByDisplay
+                $changedByDisplay,
+                $ticket['visibility'] ?? 'public'
             );
         }
     }