feat: notify configured recipients when something is booked

.env

@@ -56,6 +56,12 @@ MESSENGER_TRANSPORT_DSN=doctrine://default?auto_setup=0
 MAILER_DSN=null://null
 ###< symfony/mailer ###
 
+###> app/notifications ###
+# Sender of every booking notification; each configured recipient gets their own mail.
+# Must be an address the mail host accepts mail from -- production overrides this in .env.local.
+MAILER_FROM=[email protected]
+###< app/notifications ###
+
 ###> app/myep-oauth2 ###
 # OAuth2 against myep, the EP identity provider. ephub is a confidential client using the
 # authorization_code grant; it reads /api/userinfo once at login and then uses its own session,