fix: multi-worker translation status and OCR fallback handling

Translation status (multi-worker support):
- Add filesystem lock files (.translating) to track in-progress translations
- Check lock files in /status API when job_state not found in current worker
- Remove lock files on translation success or failure

OCR fallback fix:
- Fix empty pages when layout analysis fails but OCR succeeds
- Change 'enhanced_results' in ocr_results to ocr_results.get('enhanced_results')
- This ensures fallback to text_regions when enhanced_results is empty list

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

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

backend/app/services/translation_service.py

@@ -708,7 +708,14 @@ class TranslationService:
             f"Starting translation: task_id={task_id}, target={target_lang}"
         )
 
+        # Create lock file to indicate translation in progress (for multi-worker support)
+        output_filename = result_json_path.stem.replace('_result', '')
+        lock_file_path = result_json_path.parent / f"{output_filename}_translated_{target_lang}.translating"
+
         try:
+            # Create lock file
+            lock_file_path.touch()
+
             # Load source JSON
             with open(result_json_path, 'r', encoding='utf-8') as f:
                 result_json = json.load(f)
@@ -786,12 +793,19 @@ class TranslationService:
                 f"saved to {output_path}"
             )
 
+            # Remove lock file on success
+            if lock_file_path.exists():
+                lock_file_path.unlink()
+
             return True, output_path, None
 
         except Exception as e:
             logger.error(f"Translation failed: {e}")
             import traceback
             traceback.print_exc()
+            # Remove lock file on failure
+            if lock_file_path.exists():
+                lock_file_path.unlink()
             return False, None, str(e)
 
     def get_job_state(self, task_id: str) -> Optional[TranslationJobState]: