solve task execution issues (#90)

api/apps/document_app.py

api/db/services/task_service.py

api/settings.py

deepdoc/parser/pdf_parser.py

deepdoc/vision/layout_recognizer.py

deepdoc/vision/table_structure_recognizer.py

rag/app/book.py

rag/app/manual.py

rag/app/naive.py

rag/app/paper.py

rag/app/presentation.py

rag/app/resume.py

rag/nlp/__init__.py

rag/nlp/search.py

rag/svr/task_broker.py

rag/svr/task_executor.py