fix: oidc scopes are invalid when concatenated (#3332)

lib/postal/config_schema.rb

@@ -550,7 +550,7 @@ module Postal
       string :scopes do
         description "Scopes to request from the OIDC server."
         array
-        default "openid,email"
+        default ["openid", "email"]
       end
 
       string :uid_field do