fix: OCR Track reflow PDF and translation with image text filtering

- Add OCR Track support for reflow PDF generation using raw_ocr_regions.json
- Add OCR Track translation extraction from raw_ocr_regions instead of elements
- Add raw_ocr_translations output format for OCR Track documents
- Add exclusion zone filtering to remove text overlapping with images
- Update API validation to accept both translations and raw_ocr_translations
- Add page_number field to TranslatedItem for proper tracking

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/routers/translate.py

@@ -578,7 +578,10 @@ async def download_translated_pdf(
         with open(translation_file, 'r', encoding='utf-8') as f:
             translation_data = json.load(f)
 
-        if not translation_data.get('translations'):
+        # Check for translations (Direct Track) or raw_ocr_translations (OCR Track)
+        has_translations = translation_data.get('translations')
+        has_raw_ocr_translations = translation_data.get('raw_ocr_translations')
+        if not has_translations and not has_raw_ocr_translations:
             raise HTTPException(
                 status_code=status.HTTP_400_BAD_REQUEST,
                 detail="Translation file is empty or incomplete"