Get Log Column Catalog

Metadata-driven column catalog for the logs table (Metric Visibility panel).

Returns structural + standard columns (static registry, channel-filtered) and the tenant's active custom metrics (from DB) as toggleable columns. Values for standard columns are already on the log row; the frontend renders them via render.

channel is a Literal so FastAPI validates it (422 on anything else) and the allowed values surface in the OpenAPI schema.