Extract receipt data asynchronously with webhook notification

Submits a receipt for asynchronous AI extraction. Returns a job ID immediately.

When extraction completes (or fails), a webhook POST is sent to the callbackUrl with the full extraction result as JSON payload.

Webhook payload includes:

• event: "receipt.extraction.completed" or "receipt.extraction.failed"
• jobId, status, filename, durationMs
• result: full ReceiptExtractionResult (on success)
• error: error message (on failure)

Security: If callbackSecret is provided, the webhook includes an X-DocGen-Signature header with HMAC-SHA256 of the payload for verification.

Jobs are kept in memory for 30 minutes and then auto-cleaned.