SubscriptionEntityType is an int in Go but its custom MarshalJSON emits
"project"/"task" strings. Huma reflected it as an integer schema, so
AutoPatch's GET-to-PUT echo failed request validation with
"subscription.entity: expected integer" whenever the requesting user had
a task or project subscription, blocking all v2 task and project PATCH
writes for subscribed users.
Add a Schema override reflecting it as a string enum, same pattern as
ProjectViewKind and BucketConfigurationModeKind.
Fixes#3316
