feat: add receipt extraction proxy endpoint (refs #130)

Add POST /api/ocr/extract/receipt endpoint that proxies to the Python
OCR service's /extract/receipt for receipt-specific field extraction.

- ReceiptExtractionResponse type with receiptType, extractedFields, rawText
- OcrClient.extractReceipt() with optional receipt_type form field
- OcrService.extractReceipt() with 10MB max, image-only validation
- OcrController.extractReceipt() with file upload and error mapping
- Route with auth middleware
- 9 unit tests covering normal, edge, and error scenarios

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/ocr/index.ts

@@ -8,4 +8,5 @@ export type {
   JobResponse,
   JobStatus,
   OcrResponse,
+  ReceiptExtractionResponse,
 } from './domain/ocr.types';