Periodic conservative garbage collection for large-value blob storage.

The sweeper asks Router.sweep_blob_garbage/1 to build the live reference set from shard keydirs before deleting unreferenced blob files and stale tmp files. Append segments are reclaimed only when no live ref points into the whole segment, so the sweep remains conservative while still cleaning the default segment-backed blob layout. To avoid scanning keydirs on idle systems, each tick first checks blob storage stats and skips the expensive sweep only when no blob files or temporary files exist.