gc: add observability for old GC barriers

## Enhancement Task

A GC barrier left behind by a faulty or stalled service can keep transaction safe point advancement blocked for days, preventing obsolete data from being reclaimed and eventually causing disk pressure. Operators need visibility into which barriers retain old data so they can investigate the owning service and determine whether the retained history can be released safely.

No file or test is named. Start by locating the GC barrier and transaction safe point handling, then determine where operators should see barriers retaining old data and which owner or age details are available. Done means operators can identify blocking old barriers and investigate whether their retained history can be safely released.

Written by the indexing model from the issue text.