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/external/ocr-client.ts

@@ -2,7 +2,7 @@
  * @ai-summary HTTP client for OCR service communication
  */
 import { logger } from '../../../core/logging/logger';
-import type { JobResponse, OcrResponse, VinExtractionResponse } from '../domain/ocr.types';
+import type { JobResponse, OcrResponse, ReceiptExtractionResponse, VinExtractionResponse } from '../domain/ocr.types';
 
 /** OCR service configuration */
 const OCR_SERVICE_URL = process.env.OCR_SERVICE_URL || 'http://mvp-ocr:8000';
@@ -119,6 +119,62 @@ export class OcrClient {
     return result;
   }
 
+  /**
+   * Extract data from a receipt image using receipt-specific OCR.
+   *
+   * @param fileBuffer - Image file buffer
+   * @param contentType - MIME type of the file
+   * @param receiptType - Optional receipt type hint (e.g., 'fuel')
+   * @returns Receipt extraction result
+   */
+  async extractReceipt(
+    fileBuffer: Buffer,
+    contentType: string,
+    receiptType?: string
+  ): Promise<ReceiptExtractionResponse> {
+    const formData = this.buildFormData(fileBuffer, contentType);
+    if (receiptType) {
+      formData.append('receipt_type', receiptType);
+    }
+
+    const url = `${this.baseUrl}/extract/receipt`;
+
+    logger.info('OCR receipt extract request', {
+      operation: 'ocr.client.extractReceipt',
+      url,
+      contentType,
+      fileSize: fileBuffer.length,
+      receiptType,
+    });
+
+    const response = await this.fetchWithTimeout(url, {
+      method: 'POST',
+      body: formData,
+    });
+
+    if (!response.ok) {
+      const errorText = await response.text();
+      logger.error('OCR receipt extract failed', {
+        operation: 'ocr.client.extractReceipt.error',
+        status: response.status,
+        error: errorText,
+      });
+      throw new Error(`OCR service error: ${response.status} - ${errorText}`);
+    }
+
+    const result = (await response.json()) as ReceiptExtractionResponse;
+
+    logger.info('OCR receipt extract completed', {
+      operation: 'ocr.client.extractReceipt.success',
+      success: result.success,
+      receiptType: result.receiptType,
+      fieldCount: Object.keys(result.extractedFields).length,
+      processingTimeMs: result.processingTimeMs,
+    });
+
+    return result;
+  }
+
   /**
    * Submit an async OCR job for large files.
    *