feat: implement Case/When expressions for QuerySet.annotate()

Ryx currently supports CASE WHEN internally for bulk updates, but it is not exposed as a high-level API for annotate().

Goal: Implement a Case and When API allowing users to create conditional columns in their result sets.

Example usage:

posts = await Post.objects.annotate(
    status=Case(
        When(views__gt=1000, then=Value("viral")),
        When(views__gt=100, then=Value("popular")),
        default=Value("normal")
    )
)

Implementation hint:

1. Create Case, When, and Value classes in ryx/queryset.py.
2. Update the Rust compiler in ryx-query to handle these new AST nodes and generate the corresponding CASE WHEN ... THEN ... ELSE ... END SQL. Label: advanced

Start in ryx/queryset.py with the existing query expression APIs, then inspect the Rust compiler in ryx-query and its bulk-update CASE WHEN support. Implement the Case, When, and Value API and compiler handling so the annotated query produces CASE WHEN ... THEN ... ELSE ... END SQL matching the issue's example.