SMS notification templates are now checked before they can break a send

If you customize the SMS templates Subi sends your subscribers, an invalid template used to save without any warning, and only failed later when a real message tried to go out, sometimes with no message reaching the customer at all.

Saving or previewing a template now checks that it can actually render. If it can't, you get an immediate error explaining why, and the previous, working version of your template stays in place, so a mistake never overwrites the template that was still working. If a broken template somehow still exists on your account, Subi now falls back to sending the default message rather than dropping the send entirely.

We also fixed the labeling on the SMS and email template settings pages, which incorrectly told merchants the template field used Liquid; both actually use Django templating, and the pages now say so.

This is a validation and messaging fix only; how your templates render for customers is unchanged.