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/api/ocr.routes.ts

@@ -23,6 +23,12 @@ export const ocrRoutes: FastifyPluginAsync = async (
     handler: ctrl.extractVin.bind(ctrl),
   });
 
+  // POST /api/ocr/extract/receipt - Receipt-specific OCR extraction
+  fastify.post('/ocr/extract/receipt', {
+    preHandler: [requireAuth],
+    handler: ctrl.extractReceipt.bind(ctrl),
+  });
+
   // POST /api/ocr/jobs - Submit async OCR job
   fastify.post('/ocr/jobs', {
     preHandler: [requireAuth],