fix: correctly calculate undersubscription fees

src/Service/GroupsPriceCalculator.php

@@ -244,10 +244,17 @@ class GroupsPriceCalculator
             $shortTermSurcharge = (int) round(($basePrice + $additionalPersonsPrice) * self::SHORT_TERM_FACTORS[$nights] / 100);
         }
 
-        // Rule 4: undersubscription surcharge (only when board is selected)
+        // Board is only billable — and only triggers the catering surcharge — when it costs
+        // something. A free "Selbstversorgung" board is stored as 0 (BoardService::$price is
+        // non-nullable), so a plain null check would charge the surcharge for it.
+        $paidBoardPricePerPersonNight = ($boardServicePricePerPersonNight ?? 0) > 0
+            ? $boardServicePricePerPersonNight
+            : null;
+
+        // Rule 4: undersubscription surcharge (only when a paid board is selected)
         $undersubscriptionSurcharge = 0;
         $undersubscriptionThreshold = null;
-        if ($boardServicePricePerPersonNight !== null) {
+        if ($paidBoardPricePerPersonNight !== null) {
             $surcharge30 = (int) round(('CHF' === $currency ? $this->config['undersubscription30Chf'] : $this->config['undersubscription30Eur']) * 100);
             $surcharge40 = (int) round(('CHF' === $currency ? $this->config['undersubscription40Chf'] : $this->config['undersubscription40Eur']) * 100);
 
@@ -262,8 +269,8 @@ class GroupsPriceCalculator
 
         // Rule 5: board price
         $boardPrice = 0;
-        if ($boardServicePricePerPersonNight !== null) {
-            $boardPrice = $boardServicePricePerPersonNight * $effectivePax * $nights;
+        if ($paidBoardPricePerPersonNight !== null) {
+            $boardPrice = $paidBoardPricePerPersonNight * $effectivePax * $nights;
         }
 
         // Rule 6: additional services