feat: expose winter placements to n8n over an api endpoint

.env.test

@@ -10,5 +10,7 @@ APP_TRUSTED_HOSTS='^localhost$'
 # loaded in the test environment - so the throwaway key the tests send belongs here.
 # Keep it in sync with ApplicationFormControllerTest::KEY.
 APPLICATION_FORM_API_KEY=test-application-form-api-key
+# Keep in sync with Api\Assignment\ListControllerTest::KEY.
+N8N_API_KEY=test-n8n-api-key
 # The receipt mail refuses to render without it, and several tests send that mail.
 APPLICATION_FORM_BOOKING_URL=https://bookings.example.com/team-infocall