chore: major code cleanup and type safety using phpstan

tests/Service/RoomAssignerTest.php

@@ -293,6 +293,9 @@ class RoomAssignerTest extends TestCase
         self::assertNull($bookingDto->participants[1]->assignedRoomId);
     }
 
+    /**
+     * @param array<int, array{roomId: int, quantity: int|null, capacity?: int}> $selections
+     */
     private function createBookingDtoWithRoomSelections(array $selections): BookingDto
     {
         $travel = new Travel();