Two-way relation does not backfill existing links

When I enable two-way linking on an existing relation property, links that already exist are not added to the other side.

Steps:

1. Type "Member" has a relation property "Role" pointing to type "Role".
2. Several members already link to roles.
3. Enable two-way linking. Type "Role" gets the counter property "Members".

Expected: Each role lists all members that already link to it. Actual: "Members" stays empty. Only links set after enabling show up on both sides. Removing and re-adding a link fixes it for that one object.

Please backfill existing links when two-way linking is enabled.

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