minimize storage impact from logging

#122 enabled logging into a file at DEBUG level, which could come of great use for troubleshooting in case of errors.

But each log file is about 19MB and in a day we already amassed 0.5GB of them. Clearly it is a bit "wasteful" and we should do something about that. Some ideas

- keep the log only if run errored out and otherwise remove (should probably be possible to setup I bet)
- gzipping folds it to ~10% of its size. May be we should store them gzipped anyways -- why to waste space (although IIRC this partition also has file system level compression enabled)

@jwodder -- wdyt?

Review issue #122 and locate the logging configuration that enabled DEBUG-level file logging. Measure the current log growth, then determine how retention or compression should reduce accumulated storage while preserving logs for failed runs; done means the agreed behavior is implemented and verified.

Written by the indexing model from the issue text.