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/emailTemplates.ts

@@ -336,6 +336,7 @@ export const ORDER_STATUS_EMAIL_ICON: Record<string, string> = {
   "order-tracking-corrected": "📦",
   "order-delivered": "🎉",
   "payment-method-switched": "💳",
+  "back-in-stock": "🔔",
 };
 
 export function renderOrderStatusHtml(