When a sitelink is added to an item, the target wiki should immediately be subscribed to receive change notifications, be adding an entry to the wb_changes_subscription table.
Usually, entries in the wb_changes_subscription table are managed by the client itself. If we however rely on wb_changes_subscription to determine which wiki should receive change notifications, a "chicken and egg" problem presents itself: The client wiki will not be notified about the sitelink being added, because the client wiki was not yet subscribed; so the client wiki will not add a subscription.
To resolve this, the repo should add a subscription to wb_changes_subscription when a sitelink is added (before the corresponding change is written to wb_changes).
NOTE: the repo should not remove the subscription from wb_changes_subscription when a sitelink is removed, since the client wiki may still be using the item by other means, e.g. through Lua.