feat: notify configured recipients when something is booked

config/services.yaml

@@ -26,6 +26,12 @@ services:
         arguments:
             $logger: '@monolog.logger.audit'
 
+    # Booking notifications. The sender is also the To of every notification -- the actual
+    # recipients are Bcc'd -- so it has to be an address that may both send and receive.
+    App\Service\BookingNotifier:
+        arguments:
+            $sender: '%env(string:MAILER_FROM)%'
+
     # The myep OAuth2 client. Endpoint URLs are configuration rather than constants (same as
     # myep-team), so dev, staging and production differ by .env alone. Empty credentials are a
     # valid state: the login page then says the client is not registered yet instead of bouncing