query past scan results

Currently, we dump all the scans as blobs into a table, and right now we only support querying the most recent scan for a domain.

let's make it possible to be able to look up all the failures in a given time period for a particular domain or set of hostnames. This is a tracking issue for this functionality, because it's a big change.

- [ ] #132 Migrate to an "ORM"
- [ ] #140 DB schema clarification and migration

No implementation files or entry points are named. Start by reading #132 and #140 to understand the proposed ORM migration and database schema, then identify how scans are stored and queried; done means retrieving failures for a time period by domain or by a set of hostnames.

Written by the indexing model from the issue text.