feat: implement actual OCR processing in start_task endpoint

Changes:
- Add process_task_ocr background function to execute OCR processing
- Initialize OCRService and process uploaded file
- Save OCR results to JSON and Markdown files
- Update task status to COMPLETED/FAILED based on processing outcome
- Use FastAPI BackgroundTasks for async processing
- Direct database updates in background task (bypass user isolation)

Features:
- Real OCR processing with GPU/CPU acceleration
- Processing time tracking
- Error handling and status updates
- Result files saved in task-specific directories

Fixes:
- Task status stuck in PROCESSING (no actual OCR execution)
- No CPU/GPU utilization during "processing"

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

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

backend/app/routers/tasks.py

@@ -9,9 +9,11 @@ from pathlib import Path
 import shutil
 import hashlib
 
-from fastapi import APIRouter, Depends, HTTPException, status, Query, UploadFile, File
+from fastapi import APIRouter, Depends, HTTPException, status, Query, UploadFile, File, BackgroundTasks
 from fastapi.responses import FileResponse
 from sqlalchemy.orm import Session
+import json
+from datetime import datetime
 
 from app.core.deps import get_db, get_current_user
 from app.core.config import settings
@@ -29,12 +31,96 @@ from app.schemas.task import (
 )
 from app.services.task_service import task_service
 from app.services.file_access_service import file_access_service
+from app.services.ocr_service import OCRService
 
 logger = logging.getLogger(__name__)
 
 router = APIRouter(prefix="/api/v2/tasks", tags=["Tasks"])
 
 
+def process_task_ocr(task_id: str, task_db_id: int, file_path: str, filename: str):
+    """
+    Background task to process OCR for a task
+
+    Args:
+        task_id: Task UUID string
+        task_db_id: Task database ID
+        file_path: Path to uploaded file
+        filename: Original filename
+    """
+    from app.core.database import SessionLocal
+    from app.models.task import Task
+
+    db = SessionLocal()
+    start_time = datetime.now()
+
+    try:
+        logger.info(f"Starting OCR processing for task {task_id}, file: {filename}")
+
+        # Get task directly by database ID (bypass user isolation for background task)
+        task = db.query(Task).filter(Task.id == task_db_id).first()
+        if not task:
+            logger.error(f"Task {task_id} not found in database")
+            return
+
+        # Initialize OCR service
+        ocr_service = OCRService()
+
+        # Process the file with OCR
+        ocr_result = ocr_service.process_image(
+            image_path=Path(file_path),
+            lang='ch',
+            detect_layout=True
+        )
+
+        # Calculate processing time
+        processing_time_ms = int((datetime.now() - start_time).total_seconds() * 1000)
+
+        # Create result directory
+        result_dir = Path(settings.result_dir) / task_id
+        result_dir.mkdir(parents=True, exist_ok=True)
+
+        # Save JSON result
+        json_path = result_dir / f"{Path(filename).stem}_result.json"
+        with open(json_path, 'w', encoding='utf-8') as f:
+            json.dump(ocr_result, f, ensure_ascii=False, indent=2)
+
+        # Save Markdown result
+        markdown_path = result_dir / f"{Path(filename).stem}_result.md"
+        markdown_content = ocr_result.get('markdown', '')
+        with open(markdown_path, 'w', encoding='utf-8') as f:
+            f.write(markdown_content)
+
+        # Update task with results (direct database update)
+        task.result_json_path = str(json_path)
+        task.result_markdown_path = str(markdown_path)
+        task.processing_time_ms = processing_time_ms
+        task.status = TaskStatus.COMPLETED
+        task.completed_at = datetime.utcnow()
+        task.updated_at = datetime.utcnow()
+
+        db.commit()
+
+        logger.info(f"OCR processing completed for task {task_id} in {processing_time_ms}ms")
+
+    except Exception as e:
+        logger.exception(f"OCR processing failed for task {task_id}")
+
+        # Update task status to failed (direct database update)
+        try:
+            task = db.query(Task).filter(Task.id == task_db_id).first()
+            if task:
+                task.status = TaskStatus.FAILED
+                task.error_message = str(e)
+                task.updated_at = datetime.utcnow()
+                db.commit()
+        except Exception as update_error:
+            logger.error(f"Failed to update task status: {update_error}")
+
+    finally:
+        db.close()
+
+
 @router.post("/", response_model=TaskResponse, status_code=status.HTTP_201_CREATED)
 async def create_task(
     task_data: TaskCreate,
@@ -425,6 +511,7 @@ async def download_pdf(
 @router.post("/{task_id}/start", response_model=TaskResponse, summary="Start task processing")
 async def start_task(
     task_id: str,
+    background_tasks: BackgroundTasks,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_user)
 ):
@@ -434,11 +521,11 @@ async def start_task(
     - **task_id**: Task UUID
     """
     try:
-        task = task_service.update_task_status(
+        # Get task details
+        task = task_service.get_task_by_id(
             db=db,
             task_id=task_id,
-            user_id=current_user.id,
+            user_id=current_user.id
-            status=TaskStatus.PROCESSING
         )
 
         if not task:
@@ -447,7 +534,39 @@ async def start_task(
                 detail="Task not found"
             )
 
-        logger.info(f"Started task {task_id} for user {current_user.email}")
+        # Check if task is in pending status
+        if task.status != TaskStatus.PENDING:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail=f"Cannot start task in '{task.status.value}' status"
+            )
+
+        # Get task file
+        task_file = db.query(TaskFile).filter(TaskFile.task_id == task.id).first()
+        if not task_file:
+            raise HTTPException(
+                status_code=status.HTTP_404_NOT_FOUND,
+                detail="Task file not found"
+            )
+
+        # Update task status to processing
+        task = task_service.update_task_status(
+            db=db,
+            task_id=task_id,
+            user_id=current_user.id,
+            status=TaskStatus.PROCESSING
+        )
+
+        # Start OCR processing in background
+        background_tasks.add_task(
+            process_task_ocr,
+            task_id=task_id,
+            task_db_id=task.id,
+            file_path=task_file.stored_path,
+            filename=task_file.original_name
+        )
+
+        logger.info(f"Started OCR processing task {task_id} for user {current_user.email}")
         return task
 
     except HTTPException: