Records written before the visibility field existed carry none; the new
fail-closed authorization treated missing visibility as invalid, making every
legacy group unlistable and unreadable. Add resolved_visibility(group) that
derives effective visibility for legacy records only (owner present => private,
absent => public), leaves explicit-malformed visibility fail-closed (None), and
never derives demo/hidden. Apply it at every list, authorization, chat, and
analytics boundary while keeping demo and hidden-preview paths raw and
owner_user_id-based private isolation intact. No persisted data is rewritten.
Backend full suite passes 517; frontend 26/26; production build passes.
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