fix(backend): replace $contains with post-filter for webhook events array query

$contains generates CONTAINS() SQL in Cosmos (string match), not
ARRAY_CONTAINS() needed for string[] fields. Use findMany + in-memory
post-filter for correct behavior across both providers.

backend/src/modules/webhooks/repository.ts

@@ -96,10 +96,12 @@ export async function findSubscriptionsForEvent(
   productId: string,
   eventType: WebhookEventType
 ): Promise<WebhookSubscriptionDoc[]> {
-  // events is a string[] — $contains works for primitive arrays in memory provider
-  return subsCollection().findMany({
-    filter: { userId, productId, active: true, events: { $contains: eventType } },
+  // $contains generates CONTAINS() SQL (string match) in Cosmos, not ARRAY_CONTAINS().
+  // Use findMany + post-filter for correct behavior across both providers.
+  const subs = await subsCollection().findMany({
+    filter: { userId, productId, active: true },
   });
+  return subs.filter(s => s.events.includes(eventType));
 }
 
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