Schedule periodic maintenance on acralyzer host

Went to clean up the acralyzer instance and it was a bit of a mess - these items need periodic scheduling:

- purge ACRA reports older than X days (30 is the default, 90 seems to fit us better, not sure how to do this via an API or command line)
- mysql needs maintenance to reclaim space post-purge (`mysqlcheck -u root -p --auto-repair --optimize --all-databases`) - requires a root mysql password but it's empty in our instance 😬 so anyway just hitting return at the password prompt works)

Consequence of not doing this is that the 25GB of available storage fills - that's important to mention.

The disk appears to fill about once every 6 months