Merge pull request #28639 from overleaf/kh-analytics-for-custom-checkout-part-2

[web] support `payment-page-form-*` events for Stripe custom checkout

GitOrigin-RevId: cff1206fe0e9e03fa1c822f02ecca18f7ca27f59

services/web/types/stripe/metadata.ts

@@ -18,4 +18,5 @@ export type SubscriptionMetadata = Stripe.Metadata & {
   itm_campaign?: string
   itm_content?: string
   itm_referrer?: string
+  checkoutSource?: 'hosted-checkout' | 'elements-checkout'
 }