fix: update contract query to filter by signature date instead of started date

app/Services/StudentService.php

@@ -31,7 +31,7 @@ public function getAll(User $user, array $filters = []): Collection
                     $contractQuery->latestVersion()->where('status', 'active');
 
                     if ($endDate) {
-                        $contractQuery->where('started_date', '<=', $endDate);
+                        $contractQuery->where('signature_date', '<=', $endDate);
                     }
 
                     if ($startDate) {