Update case

Apply a seller action to an existing case.

Use this endpoint for case workflow actions like accepting/rejecting outcomes or escalating.

Rules:

• Seller must own the order associated with the case.
• Returns 404 for missing/non-owned cases.
• Returns 409 when an outcome has already been accepted and cannot be changed.

Side effects:

• Escalation actions trigger case update notifications.
• Returns the refreshed case representation after mutation.