Improve ORM enum ergonomics for feature health choices

[emyller]

FeatureHealthEventType and FeatureHealthProviderName subclass base models.Choices, so their members aren't str and depend on .value unwrapping to evaluate correctly in DB queries. Normalise to TextChoices (as done for TagType in #7877).

Acceptance criteria

• Both evaluate correctly in queries without .value unwrapping.
• Stored values unchanged.
• Tests exercise the members directly.