Under READ COMMITTED, two concurrent resolves of different comments in
the same thread could each miss the other's uncommitted resolution in
ClearOtherThreadResolutions and commit two resolved rows. Take a FOR
UPDATE lock on the thread root in its own statement before clearing, so
the loser blocks and its clear re-runs on a fresh snapshot that sees the
winner's committed resolution.
Also updates the stale loadCommentForActor docstring that still
described the backend as a plain setter with frontend-derived
resolution — the single-resolution invariant is now enforced on write.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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