debt: implement automated cache eviction for soil ID cache

[shrouxm]

Description

Currently the SoilIDCache table has no automated cache eviction, so it will slowly fill up over time. We should add data deletion commands to do some combination of:

• delete entries not associated with a site
• delete entries older than some cutoff time
• delete all entries

We're not in a rush to do this because it's possible we'll be switching architectures for offline support anyway before the cache gets too full.

Additional context

See e.g. the clean_up_deleted_files command for an example of defining a command, and this file for an example of triggering it to run regularly.