feat: add backend OCR client method for VIN decode (refs #225)

Add VinDecodeResponse type and OcrClient.decodeVin() method that sends
JSON POST to the new /decode/vin OCR endpoint. Unlike other OCR methods,
this uses JSON body instead of multipart since there is no file upload.

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

backend/src/features/ocr/domain/ocr.types.ts

@@ -131,3 +131,21 @@ export interface ManualJobResponse {
   result?: ManualExtractionResult;
   error?: string;
 }
+
+/** Response from VIN decode via Gemini (OCR service) */
+export interface VinDecodeResponse {
+  success: boolean;
+  vin: string;
+  year: number | null;
+  make: string | null;
+  model: string | null;
+  trimLevel: string | null;
+  bodyType: string | null;
+  driveType: string | null;
+  fuelType: string | null;
+  engine: string | null;
+  transmission: string | null;
+  confidence: number;
+  processingTimeMs: number;
+  error: string | null;
+}

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, ManualJobResponse, OcrResponse, ReceiptExtractionResponse, VinExtractionResponse } from '../domain/ocr.types';
+import type { JobResponse, ManualJobResponse, OcrResponse, ReceiptExtractionResponse, VinDecodeResponse, VinExtractionResponse } from '../domain/ocr.types';
 
 /** OCR service configuration */
 const OCR_SERVICE_URL = process.env.OCR_SERVICE_URL || 'http://mvp-ocr:8000';
@@ -373,6 +373,55 @@ export class OcrClient {
     return result;
   }
 
+  /**
+   * Decode a VIN string into structured vehicle data via Gemini.
+   *
+   * Unlike other OCR methods, this sends JSON (not multipart) because
+   * VIN decode has no file upload.
+   *
+   * @param vin - 17-character Vehicle Identification Number
+   * @returns Structured vehicle data from Gemini decode
+   */
+  async decodeVin(vin: string): Promise<VinDecodeResponse> {
+    const url = `${this.baseUrl}/decode/vin`;
+
+    logger.info('OCR VIN decode request', {
+      operation: 'ocr.client.decodeVin',
+      url,
+      vin,
+    });
+
+    const response = await this.fetchWithTimeout(url, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ vin }),
+    });
+
+    if (!response.ok) {
+      const errorText = await response.text();
+      logger.error('OCR VIN decode failed', {
+        operation: 'ocr.client.decodeVin.error',
+        status: response.status,
+        error: errorText,
+      });
+      const err: any = new Error(`OCR service error: ${response.status} - ${errorText}`);
+      err.statusCode = response.status;
+      throw err;
+    }
+
+    const result = (await response.json()) as VinDecodeResponse;
+
+    logger.info('OCR VIN decode completed', {
+      operation: 'ocr.client.decodeVin.success',
+      success: result.success,
+      vin: result.vin,
+      confidence: result.confidence,
+      processingTimeMs: result.processingTimeMs,
+    });
+
+    return result;
+  }
+
   /**
    * Check if the OCR service is healthy.
    *