Canonical list of signal types served by the API. This endpoint is
the single source of truth for what types are accepted by other
endpoints' signal_types filter parameters.

The default response also carries:

• a types metadata array — per-type label, association
  (company/contact), description, and content_filterable (present
  and true on the social + podcast types that accept the search
  endpoints' content_filters param; any other type returns 400 with
  it);
• a defaults object — server-side defaults applied when a filter is
  omitted (currently detected_after_days: searches without a
  detected_after only scan this many trailing days);
• a static subtypes map — the known signal_subtype vocabulary per
  type, the valid values for the search endpoints' signal_subtypes
  filter.

No include_counts needed for any of the above: it is registry data
and costs nothing to serve. With include_counts=true, each type
object carries the same subtypes list inline alongside live row
counts.

Credit cost: free.