Returns signals for a contact identified by contact_email or
contact_linkedin_url.

Note: contact-level coverage is currently low across most signal
types. Expect empty contact_signals for many inputs today. To get
company-level signals for the contact's employer, call
POST /v1/companies/enrich with the resolved domain.

Subtype filtering

Pass signal_subtypes (array, max 25 — OR semantics) to narrow to
specific subtypes, e.g. only jobChange within work-milestones.
The list is CROSS-TYPE: it applies across all requested
signal_types. 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).

Credit cost: 2 per returned signal.