Make ability score percent calculation resilient against negative values

[Oaphi]

This PR makes our ability score percent calculation more resilient against incorrect values, specifically against negative thresholds. The new helper method is intended to be expanded in the future to cover other cases (such as setting score thresholds to a value higher than 1.0).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.49%. Comparing base (1ff2d8b) to head (f839259).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	74.55% <ø> (ø)
helpers	84.91% <ø> (ø)
jobs	79.24% <ø> (ø)
models	89.97% <100.00%> (+0.01%)	⬆️
tasks	61.11% <ø> (ø)

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[cellio]

LGTM. As noted, we're not fixing the frontend here, just the backend, so you can set bad values and we'll handle them without causing server errors.