Add logic for generating background scans

`_get_required_backgrounds` should be updated to actually generate a background scan based on an experiment. It should generate a background with the same capillary type of the experiment, and at the right temperatures.

## Acceptance Criteria
- Logic is added so the correct backgrounds scan is added for each experiement

Locate `_get_required_backgrounds` and read how experiments, capillary types, and temperatures are represented. Trace the existing background-selection flow and verify that each experiment receives a background with the matching capillary type and required temperatures. Done means the correct background scan is added for every experiment described by the existing logic.

Written by the indexing model from the issue text.