Flip the cron from every-3-hours to every-hour now that a full
f00-f18 chain runs in ~45-60 min instead of ~170 min. With the
:propagation queue's concurrency-of-2, a slow chain won't block
the next one — they interleave, and the file store's last-writer
-wins semantics give the newer run_time's analysis data naturally.
Add ScoresFile.retain_window/2 that deletes any file whose
valid_time falls outside [run_time, run_time + 18h]. Call it at
fh=18 of the chain so leftover files from the previous chain
(specifically the old f00 whose valid_time the new chain doesn't
overwrite because it advanced by one hour) are discarded
immediately instead of waiting for the 2h prune cron.
The step transition path is extracted to handle_step_transition/2
to keep the nesting under credo's limit and make the final-step
logic easier to read.