Identify API

Attach anonymous device history and profile properties to a stable external user ID.

POST /api/v1/identify

{
  "userId": "user-42",
  "anonymousId": "device-abc",
  "properties": { "plan": "pro", "locale": "en-IN" }
}

Identity resolution is database-atomic. Do not recreate it as a read-then-write sequence in application code.