Ingest Segment messages

Accepts Segment-style `track`, `identify`, `group`, or batch payloads. Batches may be a `{ batch: [...] }` envelope or a bare array (as sent by Segment's Webhooks (Actions) destination with batching enabled). A batch may be up to 512 KB and 1000 messages; each message, alone or in a batch, is limited to 16 KB (an oversized message inside a batch is listed in `rejected`). Every message counts toward the workspace write limit of 100 per 10 seconds. Segment is a compatibility ingress path; after mapping, messages use the same internal ingest core as native Atune capture. `track` and `identify` may omit the account when `userId` is set. Atune then uses the account the user was most recently grouped into. For a user with no account, `identify` creates none and keeps the traits until one does; `track` makes the user an account of one. Account IDs may be strings or numbers, and `company_id`, `account_id` and `group_id` are read too. In a batch, invalid messages are listed in `rejected` and the valid ones are still stored.

curl -X POST https://test.useatune.com/api/v1/segment \  -H "Authorization: Bearer <YOUR_API_KEY>" \  -H "Content-Type: application/json" \  -d '{    "type": "track",    "messageId": "seg-msg-001",    "event": "Invoice Sent",    "userId": "user_123",    "properties": {      "accountId": "acme",      "amount": 1200    }  }'

curl -X POST "https://example.com/api/v1/segment" \  -H "Content-Type: application/json" \  -d '{    "type": "track",    "messageId": "seg-msg-001",    "event": "Invoice Sent",    "userId": "user_123",    "properties": {      "accountId": "acme",      "amount": 1200    }  }'

{  "ok": true,  "deduped": true,  "source": "segment",  "account": {    "id": "cm123",    "externalId": "acme"  },  "user": null}