Purge executions & keep statistics

# Purge executions & keep statistics

## Context

Executions are never deleted.
A way to automatically remove from database old executions would be welcome to not overflow the database.
BUT, it is interesting to keep execution statistics for history and trend purposes.

## Expected
Several possibilities:
* Remove executions older than X days/weeks/months
* Remove all executed_scenario lines of older executions, but keep the execution for statistics purposes. Mark such executions as "ARCHIVED", to avoid to needlessly join them when doing all other SQL queries
* A mix of both solutions (configurable per project, with sensitive defaults):
** Keep **all** executions of the last X days (eg. 7 days)
** After than, keep **one execution per day** and per cycle (remove all others) of the last Y days/weeks (eg. 1 month)
** After than, keep **one execution per week** and per cycle (remove all others) of the last Z days/weeks (eg. 2 months)
** After than, keep **one execution per month** and per cycle (remove all others) of the last A days/weeks (eg. 6 months)

The issue proposes several configurable retention strategies but names no files, tests, or entry points. Start by locating the execution and statistics persistence code, then determine which policy and defaults are accepted; done means old execution data is purged while the required historical statistics remain available.

Written by the indexing model from the issue text.