feat: optimize task file generation and add visualization download

Backend changes:
- Disable PP-Structure debug file generation by default
- Separate raw_ocr_regions.json generation from debug flag (critical file)
- Add visualization folder download endpoint as ZIP
- Add has_visualization field to TaskDetailResponse
- Stop generating Markdown files
- Save translated PDFs to task folder with caching

Frontend changes:
- Replace JSON/MD download buttons with PDF buttons in TaskHistoryPage
- Add visualization download button in TaskDetailPage
- Fix Processing page task switching issue (reset isNotFound)

Archives two OpenSpec proposals:
- optimize-task-files-and-visualization
- simplify-frontend-add-billing

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

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

backend/app/services/unified_document_exporter.py

@@ -255,14 +255,8 @@ class UnifiedDocumentExporter:
             logger.error(f"Failed to export JSON: {e}")
             results['json'] = None
 
-        # Export Markdown
-        try:
-            md_path = output_dir / f"{file_id}_output.md"
-            UnifiedDocumentExporter.export_to_markdown(document, md_path)
-            results['markdown'] = md_path
-        except Exception as e:
-            logger.error(f"Failed to export Markdown: {e}")
-            results['markdown'] = None
+        # Markdown export removed - no longer generating _output.md files
+        results['markdown'] = None
 
         # Export plain text
         try:
@@ -469,13 +463,13 @@ def save_unified_document(
         document: The UnifiedDocument to save
         output_dir: Output directory
         file_id: Base filename
-        formats: List of formats to export (default: ['json', 'markdown'])
+        formats: List of formats to export (default: ['json'])
 
     Returns:
         Dictionary mapping format names to output paths
     """
     if formats is None:
-        formats = ['json', 'markdown']
+        formats = ['json']
 
     results = {}
     output_dir = Path(output_dir)
@@ -488,9 +482,9 @@ def save_unified_document(
                 UnifiedDocumentExporter.export_to_json(document, path)
                 results['json'] = path
             elif fmt == 'markdown':
-                path = output_dir / f"{file_id}_output.md"
-                UnifiedDocumentExporter.export_to_markdown(document, path)
-                results['markdown'] = path
+                # Markdown export removed - skip silently
+                results['markdown'] = None
+                continue
             elif fmt == 'text':
                 path = output_dir / f"{file_id}_text.txt"
                 UnifiedDocumentExporter.export_to_text(document, path)