Replace Field(description=...) with proper docstrings in auth models

From the V2 checklist in #1253 (under "Refactor the auth way of using it"):

Drop the use of Field(description=...), and use proper docstrings. As far as I know, we are not creating json schemas from it, so it's useless.

The auth-related Pydantic models currently use Field(description=...) to document fields. Since we are not generating JSON schemas from these models, the descriptions are not surfaced anywhere useful. They should be replaced with standard Python docstrings that are more discoverable via IDEs and help().

This is part of the broader auth refactor planned for V2.

Ref: Kludex's V2 checklist in #1253

Commencez par localiser les modèles Pydantic liés à auth qui utilisent Field(description=...), puis consultez la checklist V2 dans #1253 pour connaître le périmètre prévu. Remplacez ces descriptions de champs par des docstrings Python standard et vérifiez qu’aucun des modèles auth ciblés n’utilise encore ce modèle de documentation.