Remove one creator from the shop's do-not-contact list

Un-blacklist a single creator by handle, making them eligible for outreach again.

The handle is normalized the same way as on add, so @Creator and creator both resolve to the same entry. A handle that is not on this shop's list returns 404 — this route is not a silent no-op, so a typo surfaces instead of being mistaken for a successful removal. To empty the whole list use POST /shops/blacklist/clear.

Requires a key with read_write scope and a single shop.