6 Month timeframe for storing files

Expected: index file per month, after 6 months, degenerate the files

Find way to have 1 month worth of files in one index file, and query that using what we currently have, and so on. At most 6 index files at one point in time.

Start by locating the current index-file creation and query implementation referenced by the issue. Trace how files are grouped and queried, then define the monthly retention behavior: one index per month, with no more than six index files retained and older files degenerated after six months. Done means existing queries still work across the retained monthly indexes.

Written by the indexing model from the issue text.