fix: Use LEFT JOIN in DependencyModel queries

Makes queries more defensive - returns dependencies even if the
linked ticket was deleted.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/DependencyModel.php

@@ -18,7 +18,7 @@ class DependencyModel {
     public function getDependencies($ticketId) {
         $sql = "SELECT d.*, t.title, t.status, t.priority
                 FROM ticket_dependencies d
-                JOIN tickets t ON d.depends_on_id = t.ticket_id
+                LEFT JOIN tickets t ON d.depends_on_id = t.ticket_id
                 WHERE d.ticket_id = ?
                 ORDER BY d.dependency_type, d.created_at DESC";
 
@@ -51,7 +51,7 @@ class DependencyModel {
     public function getDependentTickets($ticketId) {
         $sql = "SELECT d.*, t.title, t.status, t.priority
                 FROM ticket_dependencies d
-                JOIN tickets t ON d.ticket_id = t.ticket_id
+                LEFT JOIN tickets t ON d.ticket_id = t.ticket_id
                 WHERE d.depends_on_id = ?
                 ORDER BY d.dependency_type, d.created_at DESC";