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

Beginne damit, die auth-bezogenen Pydantic-Modelle zu finden, die Field(description=...) verwenden, und prüfe anschließend die V2-Checkliste in #1253 auf den vorgesehenen Umfang. Ersetze diese Feldbeschreibungen durch standardmäßige Python-Docstrings und stelle sicher, dass alle anvisierten auth-Modelle dieses Dokumentationsmuster nicht mehr verwenden.