Enhance knowledge base functionality and integrate S3 storage support

- Add new models for `KnowledgeDocument` and `KnowledgeChunk` to manage document ingestion and chunking.
- Implement S3-compatible storage integration for knowledge documents, allowing for file uploads and retrieval.
- Introduce API endpoints for managing knowledge bases and documents, including creation, deletion, and searching.
- Update frontend components to support knowledge base configuration and document management, improving user interaction.
- Enhance backend services for knowledge processing and retrieval, ensuring robust handling of document statuses and errors.

backend/models.py

@@ -75,6 +75,7 @@ class AssistantConfig(BaseModel):
 
     # Prompt assistant reusable tools. Execution remains type-specific in the pipeline.
     tools: list[RuntimeTool] = Field(default_factory=list)
+    knowledge_base_id: str | None = None
 
     # workflow 类型:节点图(nodes/edges)。非 workflow 为空,引擎据此决定是否启用。
     graph: dict = {}