Allow changing the status of an already-reviewed flagged message through the Moderation API

When a flagged message has been reviewed (approved or disapproved) through PATCH /moderation/flagged-messages/{id}, its status can't be changed again. Any later request returns ERR_MESSAGE_ALREADY_REVIEWED. So if a moderator disapproves a message by mistake, the customer has no way to approve it, and both users stay unable to see it.

Customer case (ticket #45777)

- A message was flagged, and the customer's moderator disapproved it by mistake (App ID 1678196cd92f69192, Message ID 12163).

- The customer tried to approve it with PATCH /moderation/flagged-messages/{id} and got ERR_MESSAGE_ALREADY_REVIEWED.

- We approved the message manually as a one-time exception. The customer confirmed it worked and asked: "It would be great to be able to do that from the API."

Requested change

Let the Moderation API approve a message that was already disapproved, for example by allowing PATCH /moderation/flagged-messages/{id} to change the status of a reviewed message. When a disapproved message is approved, it should be delivered and visible to both users, like a message that was approved in the first place.

Why it's needed

- Customers can fix wrong moderation decisions themselves, without contacting support.

- It removes the manual one-time exceptions our team handles today.

Open questions

- Should approved messages also be allowed to change to disapproved?

- Should status changes after review be recorded in an audit trail?