Webhook event returns admin_api in triggered_by field for actions performed in Portal

I tried working with Authgear webhooks recently and I made the following observation:
Although, [this file](https://github.com/authgear/authgear-server/blob/a786b95a4486a2979b1fc209bd86ab9a1690ba49/pkg/api/event/context.go#L17-L26) contains the `system` and `portal` as source of event (`triggered_by`), while testing some webhooks, I got the old `admin_api` as source for events like adding and deleting a user via the Portal.

I was expecting `triggered_by: 'portal'`

@louischan @tung2744

Start with pkg/api/event/context.go, especially the source definitions linked in the issue, then trace the webhook events emitted when users are added or deleted through the Portal. Confirm why those events still report admin_api and update the event source so Portal actions report portal; verify both event cases produce the expected triggered_by value.

Written by the indexing model from the issue text.