Check if a source-specific ID (Slack channel, Stripe customer) or universal ID (domain, email) is already linked to an entity in ackDB. Always returns 200 - 'not found' is a valid lookup result, not an error.

The resolve-once pattern:

1. First event from a source (e.g., Slack channel C04ABC123) → workflow resolves the company domain, ingests with domain identity link
2. Subsequent events from the same channel → workflow calls GET /entities/lookup?externalIdType=slack_channel_id&externalIdValue=C04ABC123&source=slack → gets the entityId back → ingests with entityId directly, skipping domain resolution

Three possible outcomes:

• Found: Identity link exists → returns entity ID, name, domain, and link timestamp
• Unresolved: ID exists in unresolved content pool (ingested without domain) → returns unresolved content IDs
• Never seen: ID not in ackDB at all → returns match: null, unresolvedMatches: []

{
  "data": {
    "match": {
      "entityId": "01HXYZ123456789ABCDEFGH",
      "entityName": "acme.com",
      "domain": "acme.com"
    }
  }
}