Add a brand or product to the shop's watchlist

Favorite one competitor brand or one product so it surfaces in the shop's Social Intelligence watchlist.

Idempotent: favoriting something already on the list is a no-op that still returns 200, so a client does not have to read the list first. The ID is not validated against the SI catalog — an unknown ID is stored and simply resolves to nothing on read.

Favorites are per shop, so this is visible to everyone on the account. Requires read_write scope. Single-shop only.