Move to Jinja templating

Some elements of processing are becoming unwieldy with standard string interpolation.

We may need to move the templating over to use Jinja.

Start by reviewing the existing processing code that uses standard string interpolation; the issue does not name specific files, entry points, or tests. Determine which templating paths are unwieldy and whether a Jinja migration is appropriate. Done requires an agreed scope and implementation plan, but the issue does not define the expected behavior or validation.

Written by the indexing model from the issue text.