feat: offer the questionnaire for placements ended within a rolling window

tests/Config/QuestionnaireCatalogTest.php

@@ -112,6 +112,18 @@ class QuestionnaireCatalogTest extends TestCase
         $this->assertSame('Alte Frage', $catalog->get('sample', 1)->getQuestion('answer')->label);
     }
 
+    public function testTheWindowStartsTheGivenDaysBeforeToday(): void
+    {
+        $this->assertEquals(new \DateTimeImmutable('today -2 days'), QuestionnaireCatalog::windowStart(2));
+    }
+
+    public function testANegativeWindowFailsLoudly(): void
+    {
+        $this->expectException(\InvalidArgumentException::class);
+
+        QuestionnaireCatalog::windowStart(-1);
+    }
+
     public function testUnknownQuestionnaireIsRejected(): void
     {
         $this->expectException(\InvalidArgumentException::class);