feat: add receipt proxy tier guard, 422 forwarding, and tests (refs #139)

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

backend/src/features/ocr/external/ocr-client.ts

@@ -159,7 +159,9 @@ export class OcrClient {
         status: response.status,
         error: errorText,
       });
-      throw new Error(`OCR service error: ${response.status} - ${errorText}`);
+      const err: any = new Error(`OCR service error: ${response.status} - ${errorText}`);
+      err.statusCode = response.status;
+      throw err;
     }
 
     const result = (await response.json()) as ReceiptExtractionResponse;