Show only changed fields (delta) in ticket activity timeline

Before: entire ticket data was logged and shown in the activity tab.
After: compare old vs new values before saving; log only fields that
actually changed as { field: { from: '...', to: '...' } } pairs.

- TicketController.php: fetch old ticket before update, compute delta
- api/update_ticket.php: same fix for the API endpoint (currentTicket
  already fetched for auth, reuse it for delta comparison)
- TicketView.php: render delta format as "Field: old → new" with color;
  truncate long values (description) at 60 chars; keep legacy flat format
  as fallback for older log entries

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/update_ticket.php

@@ -180,9 +180,20 @@ try {
                 $this->ticketModel->updateVisibility($id, $data['visibility'], $visibilityGroups, $this->userId);
             }
 
-            // Log ticket update to audit log
+            // Log ticket update to audit log — only the changed fields (delta)
             if ($this->userId) {
-                $this->auditLog->logTicketUpdate($this->userId, $id, $data);
+                $trackFields = ['title', 'priority', 'status', 'description', 'category', 'type'];
+                $delta = [];
+                foreach ($trackFields as $field) {
+                    $oldVal = (string)($currentTicket[$field] ?? '');
+                    $newVal = (string)($updateData[$field] ?? '');
+                    if ($oldVal !== $newVal) {
+                        $delta[$field] = ['from' => $oldVal, 'to' => $newVal];
+                    }
+                }
+                if (!empty($delta)) {
+                    $this->auditLog->logTicketUpdate($this->userId, $id, $delta);
+                }
             }
 
             return [