db: audit ctx.Background() uses, adopt cancellation on Close()

loadNewFileStats might take a long time if stats collection is running behind (e.g. sstables are backed by object storage). We only check `d.closed` in the beginning, then try to process all updated files:

https://github.com/cockroachdb/pebble/blob/d8d0e1e90a46148e20919811399d3d7ff837dbf3/table_stats.go#L165

Jira issue: PEBBLE-1346

Start with loadNewFileStats in table_stats.go around the linked lines and audit the ctx.Background() uses requested by the issue. Trace the stats-collection path and Close() handling to identify work that should observe cancellation. Done means the relevant background work is cancellable when the database closes, with tests covering the behavior.

Written by the indexing model from the issue text.