fix(ticketing): resolve 500 error on ticket creation

nextTicketNumber() was using count()+1 which collides when tickets
have been deleted. Now queries MAX number via SUBSTRING ordering so
the generated ticket number is always unique.

src/Models/TicketingGroup.php

@@ -36,7 +36,13 @@ class TicketingGroup extends Model
 
     public function nextTicketNumber(): string
     {
-        $count = $this->tickets()->count() + 1;
-        return $this->prefix . '-' . str_pad($count, 4, '0', STR_PAD_LEFT);
+        $prefix = $this->prefix . '-';
+        $last = $this->tickets()
+            ->where('number', 'like', $prefix . '%')
+            ->orderByRaw('CAST(SUBSTRING(number, ?) AS UNSIGNED) DESC', [strlen($prefix) + 1])
+            ->value('number');
+
+        $next = $last ? ((int) substr($last, strlen($prefix))) + 1 : 1;
+        return $prefix . str_pad($next, 4, '0', STR_PAD_LEFT);
     }
 }