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/routers/tasks.py

@@ -10,9 +10,11 @@ import shutil
 import hashlib
 
 from fastapi import APIRouter, Depends, HTTPException, status, Query, UploadFile, File, BackgroundTasks
-from fastapi.responses import FileResponse
+from fastapi.responses import FileResponse, StreamingResponse
 from sqlalchemy.orm import Session
 import json
+import zipfile
+import io
 from datetime import datetime
 
 from app.core.deps import get_db, get_current_user
@@ -413,8 +415,6 @@ async def get_task(
     processing_track = None
     if task.result_json_path:
         try:
-            import json
-            from pathlib import Path
             result_path = Path(task.result_json_path)
             if result_path.exists():
                 with open(result_path) as f:
@@ -430,9 +430,18 @@ async def get_task(
         except Exception:
             pass  # Silently ignore errors reading the result file
 
-    # Create response with processing_track
+    # Check for visualization folder (OCR Track only)
+    has_visualization = False
+    result_dir = Path(settings.result_dir) / task_id
+    visualization_dir = result_dir / "visualization"
+    if visualization_dir.exists() and visualization_dir.is_dir():
+        png_files = list(visualization_dir.glob("*.png"))
+        has_visualization = len(png_files) > 0
+
+    # Create response with processing_track and has_visualization
     response = TaskDetailResponse.model_validate(task)
     response.processing_track = processing_track
+    response.has_visualization = has_visualization
     return response
 
 
@@ -1198,6 +1207,87 @@ async def download_unified(
         )
 
 
+@router.get("/{task_id}/download/visualization", summary="Download visualization images as ZIP")
+async def download_visualization_zip(
+    task_id: str,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    Download visualization images as ZIP file.
+
+    Only available for OCR Track tasks with visualization folder.
+    Returns a ZIP file containing all PNG images from the visualization folder.
+
+    - **task_id**: Task UUID
+    """
+    try:
+        # Get task details
+        task = task_service.get_task_by_id(
+            db=db,
+            task_id=task_id,
+            user_id=current_user.id
+        )
+
+        if not task:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail="Task not found"
+            )
+
+        if task.status != TaskStatus.COMPLETED:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail="Task not completed"
+            )
+
+        # Check for visualization folder
+        result_dir = Path(settings.result_dir) / task_id
+        visualization_dir = result_dir / "visualization"
+
+        if not visualization_dir.exists() or not visualization_dir.is_dir():
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail="Visualization folder not found. This feature is only available for OCR Track tasks."
+            )
+
+        # Get all PNG files
+        png_files = list(visualization_dir.glob("*.png"))
+        if not png_files:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail="No visualization images found"
+            )
+
+        # Create ZIP in memory
+        zip_buffer = io.BytesIO()
+        with zipfile.ZipFile(zip_buffer, 'w', zipfile.ZIP_DEFLATED) as zip_file:
+            for png_file in sorted(png_files):
+                # Add file with relative name (just the filename)
+                zip_file.write(png_file, png_file.name)
+
+        zip_buffer.seek(0)
+
+        logger.info(f"Created visualization ZIP for task {task_id} with {len(png_files)} images")
+
+        return StreamingResponse(
+            zip_buffer,
+            media_type="application/zip",
+            headers={
+                "Content-Disposition": f"attachment; filename={task_id}_visualization.zip"
+            }
+        )
+
+    except HTTPException:
+        raise
+    except Exception as e:
+        logger.exception(f"Failed to download visualization for task {task_id}")
+        raise HTTPException(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            detail=f"Failed to download: {str(e)}"
+        )
+
+
 # ===== Preprocessing Preview Endpoints =====
 
 @router.post("/{task_id}/preview/preprocessing", response_model=PreprocessingPreviewResponse, summary="Preview preprocessing effect")