Experiment.protocol_description is a JSONField

### Context

I was digging through the experiment model while updating the SRA parser and was curious about default values when I noticed that the Experiment.protocol_description is a JSONField column in the database which has an empty dict as the default value.

### Problem or idea

Depending on the results from looking further into how this is used, we should either update the column type to match the content (TextField) or store all `protocol_descriptions` as JSON values, whichever is more appropriate. We are most likely incurring a minor performance penalty. Either way having strings in a JSONField could lead to errors down the line.

### Solution or next step

- Check to see if we actually use this field as a JSON field for any experiments / sources.
- If we don't we should migrate the field to a `TextField`.
- If we do, we should migrate so that all `protocol_descriptions` are JSON values

Locate the Experiment model and search for protocol_description and protocol_descriptions usages across the project. Inspect existing stored values and database migrations to determine whether the field is used as JSON; done means the column type and stored values consistently match the chosen representation, with migration coverage for existing data.

Written by the indexing model from the issue text.