Create mixed bulk job

Enqueues screenshots or scrapes in a single batch. Provide `urls` (simple list) or `items` (per-item config), not both. Webhook deliveries include `X-ByteKit-Event: bulk.completed` so receivers can dispatch on the header without parsing the body shape. **Limits:** request bodies are guarded in a two-tier shape. A streamed body with no `Content-Length` header is capped at 4 MB before your API key is checked; a body that declares `Content-Length` is instead bounded there by a 32 MB fail-safe (not the real limit — see below), since that branch reads nothing until your key is verified. Once your key is verified, a second check re-gates the body against your account's own `max_bulk_size` (at this endpoint's richer per-item override budget), which reaches 28.7 MB for the largest documented account. A standard-plan account's effective ceiling is 4 MB; either tier's overrun returns a `400` with error code `request_too_large`.

curl -X POST https://api.bytekit.com/v1/bulk \
  -H "Authorization: Bearer sk_live_your_api_key_here" \
  -H "Content-Type: application/json" \
  -d '{"items":[{"url":"https://example.com","type":"screenshot","device":"mobile","format":"png","quality":90,"full_page":true,"dark_mode":true,"viewport":{"width":1280,"height":720}},{"url":"https://example.org","type":"scrape","formats":["markdown","html"],"include_html":true,"wait_until":"networkidle","wait_for_selector":"#main","delay_ms":500,"metadata":{"item_tag":"hero"}}],"defaults":{"type":"screenshot","device":"desktop","country":"US","formats":["rawHtml"],"device_scale_factor":2},"metadata":{"campaign":"q3"},"webhook_url":"https://hooks.example.com/bytekit"}'

{
  "id": "bulk_01j9abc123",
  "status": "processing",
  "total": 0,
  "completed": 0,
  "failed": 0,
  "created_at": "2019-08-24T14:15:22Z",
  "completed_at": "2019-08-24T14:15:22Z",
  "total_credits_charged": 0,
  "estimated_credits": 0,
  "webhook_url": "string"
}