fix: correctly apply due dates for contract and invoice upload reminders

src/Service/Assignment/TimelineService.php

@@ -4,7 +4,7 @@ namespace App\Service\Assignment;
 
 use App\Entity\Assignment;
 use App\Model\TimelineItem;
-use Carbon\CarbonPeriod;
+use Carbon\CarbonPeriodImmutable;
 
 /**
  * Service for processing assignments into timeline visualization data.
@@ -110,13 +110,13 @@ class TimelineService
      * Returns a period from 6 weeks ago to 6 months in the future,
      * providing context for both recent and upcoming assignments.
      *
-     * @return CarbonPeriod The active time window for timeline display
+     * @return CarbonPeriodImmutable The active time window for timeline display
      */
-    public function getActiveWindow(): CarbonPeriod
+    public function getActiveWindow(): CarbonPeriodImmutable
     {
         $dateFrom = (new \DateTimeImmutable())->modify('-6 weeks');
         $dateTo = (new \DateTimeImmutable())->modify('+6 months');
 
-        return CarbonPeriod::create($dateFrom, $dateTo);
+        return CarbonPeriodImmutable::create($dateFrom, $dateTo);
     }
 }