Returns every signal we have for a given company, identified by
domain (preferred), company_name, or linkedin_url.

Subtype filtering

Pass signal_subtypes (array, max 25 — OR semantics) to narrow to
specific subtypes, e.g. only hiringSalesRoles within
hiring-trends. The list is CROSS-TYPE: it applies across all
requested signal_types, and SEC filing types share content-tag
subtypes — {"signal_types":["sec-8k","sec-10k"], "signal_subtypes":["aiInvestment"]} is a first-class pattern.
Discover valid values via the per-type subtypes array on
GET /v1/signals/types?include_counts=true. Matching is
case-insensitive against the known vocabulary; unknown values fall
back to exact match (0 results, never a 400). The singular
signal_subtype remains supported and is unioned with the array.
Signals excluded by the filter are not returned and not billed
(enrichment bills per returned signal).

Contact-association signal types

This endpoint returns company-association signals only. Contact
types (for example work-milestones) carry a person, not a company.

• If every requested signal_types value is a contact type, the
  request fails with a 400 (invalid_request). No credits are
  charged. The error names the rejected types and gives a
  POST /v1/contacts/search body with company_domains to use
  instead.
• If the request mixes company and contact types, only the company
  types are returned and billed. A warnings[] entry names the
  dropped contact types.

Credit cost: 2 per returned signal (billed per row, never on
zero-result responses).