feat: accept form submissions from the websites over an API endpoint

.env

@@ -82,6 +82,8 @@ MYEP_OAUTH2_URL_AUTHORIZE=https://my.ep-reisen.de/authorize
 MYEP_OAUTH2_URL_ACCESS_TOKEN=https://my.ep-reisen.de/token
 MYEP_OAUTH2_URL_RESOURCE_OWNER_DETAILS=https://my.ep-reisen.de/api/userinfo
 MYEP_OAUTH2_SCOPES=email,id,roles,profile
+# Shared key the sibling websites send in the X-Api-Key header. Empty here: set it per stage.
+APPLICATION_FORM_API_KEY=
 
 ###> symfony/mailjet-mailer ###
 # Bulk teamer mailings only, everything else goes out over MAILER_DSN. Set the real