Use enums instead of strings in some model fields

### Context

I just spent a long time tracking down an issue whose ultimate cause was one of the compendia tests having `sample.technology = "RNASEQ"` instead of `sample.technology = "RNA-SEQ"`.

### Problem or idea

Using strings for fields that only have a finite number of possible values can be error-prone, and it makes typos like the above hard to detect.

### Solution or next step

Switch to enums like we use in `ProcessorPipeline`. We would need to look into how Django deals with enums, because ideally we could make this change without a migration.

Start by inspecting the existing ProcessorPipeline enum and the Django model fields that have finite values; the issue does not name specific models or tests. Research Django’s enum and migration behavior, then identify the affected fields and verify that their values reject typos while avoiding a migration if feasible.

Written by the indexing model from the issue text.