Update vacation mode

Enable or disable vacation mode for the authenticated seller.

Use this when a seller needs to temporarily stop receiving new orders.

State changes:

• Updates the seller vacation flag.
• Hides all seller listings when vacation mode is enabled.
• Removes existing listing holds when vacation mode is enabled so baskets can self-clean on next access.

Access and validation:

• Seller must exist and be active.
• Returns 401 when the seller is not active.
• Returns 204 on success with no body.