Batch upsert customers by external ID

Create or update multiple customers by external system ID. Supports up to 100 customers per request with stale update protection. --- **Related endpoints** - `GET /customers/{customerId}/invoices` — List customer invoices - `GET /customers/{customerId}/subscriptions` — List customer subscriptions - `GET /customers/{customerId}/orders` — List customer orders - `GET /customers/{customerId}/payments` — List customer payments - `POST /customers` — Create a new customer - `GET /customers` — List customers - `GET /customers/{id}` — Get customer by ID - `PATCH /customers/{id}` — Update customer details **Common errors** - `400 invalid_request` — malformed payload or failed validation. - `401 authentication_error` — missing, invalid, expired, or revoked credential. Codes: `authentication_failed`, `invalid_api_key`, `expired_api_key`, `api_key_revoked`, `session_invalid`, `merchant_required`. Carries a `WWW-Authenticate: Bearer` challenge. - `403 authorization_error` — the caller is identified but the action is denied. Codes include `insufficient_permissions`, `ip_not_allowed`, `merchant_mismatch`, and `origin_not_allowed`. **Idempotency** Pass an `Idempotency-Key` header (UUID v4 recommended) to make retries safe. Keys are valid for 24 hours; see [the idempotency guide](/docs/fundamentals/idempotency).

curl -X PUT "https://api.revkeen.com/v2/customers/external/batch" \
  -H "x-api-key: $REVKEEN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "source": "practicehub",
    "customers": [
      {
        "external_id": "PAT-12345",
        "email": "john@example.com",
        "name": "John Doe",
        "phone": "+1-555-123-4567",
        "address_line1": "string",
        "address_line2": "string",
        "city": "string",
        "state": "string",
        "postal_code": "string",
        "country": "US",
        "tax_id": "string",
        "tax_id_country": "string",
        "notes": "string",
        "metadata": {},
        "external_updated_at": "2026-01-01T00:00:00Z"
      }
    ]
  }'

{  "created": 0,  "updated": 0,  "skipped": 0,  "failed": [    {      "external_id": "string",      "error": "string"    }  ]}