feat: price calendar and price table for group houses

Ports AjaxCalendarController::priceCalendarAction and priceTableAction
(MyE&P price config and price table), used on the /gruppe/gruppenhaus-*
pages and example trips. Page cache tags and "do not cache" on API
errors go through the frontend.cache.collector and
frontend.cache.instruction request attributes instead of TSFE.

Left out: priceConfig (no caller) and range (daytrip calendar via
bpn-connect, out of scope), so bpn-connect needs no credentials.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

config/system/secrets.example.php

@@ -17,9 +17,6 @@ return [
             'myEpApiClientSecret' => '',
             'myEpApiUrlAuthorize' => '',
             'myEpApiUrlToken' => '',
-            // bpn-connect (price calendar)
-            'bpnConnectApiBaseUrl' => '',
-            'bpnConnectApiKey' => '',
         ],
     ],
 ];