Fix f-string backslash escape syntax error

structured_ocr.py

@@ -1669,7 +1669,7 @@ class StructuredOCR:
                     "languages": ["English"],
                     "ocr_contents": {
                         "raw_text": ocr_markdown[:50000] if ocr_markdown else "No text could be extracted",
-                        "error": f"AI processing failed: {str(e).replace('\"', '\\\"')}"
+                        "error": "AI processing failed: " + str(e).replace('"', '\\"')
                     },
                     "processing_method": "fallback",
                     "processing_error": str(e),