Look up member by identifier

Look up a member by one of the supported identifier combinations:

• subscriber_id only: exact match on subscriber ID
• phone_number only: matches primary or alternate phone (digit-normalized)
• subscriber_id + date_of_birth: both must match the same record

Returns basic member information including support team and language.

Requires scope: members:read

Note: Uses POST to keep PHI out of URLs (compliance requirement).