Create batch job object

A batch job should be its own object stored in the database, with a reference to its parent project. This should allow querying of the status of jobs in the batch.

The issue does not name a file, entry point, schema, or test to begin with. First inspect the existing project and database models, then clarify the batch job fields, parent-project relationship, status queries, and tests needed to define when the feature is complete.

Written by the indexing model from the issue text.