Fix missing back-in-stock email Live Preview (404)

'back-in-stock' was added to the Payload-side email-templates type
but never wired into this repo's own EmailTemplateType union or the
Live Preview page's VALID_TYPES/fallback-heading maps — every other
type is registered in three places, this one was only in one, so
opening its Live Preview 404'd.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/payload.ts

@@ -897,7 +897,8 @@ export type EmailTemplateType =
   | "order-tracking-added"
   | "order-tracking-corrected"
   | "order-delivered"
-  | "payment-method-switched";
+  | "payment-method-switched"
+  | "back-in-stock";
 
 type PayloadEmailTemplate = {
   type: EmailTemplateType;