fix: Critical bugs - LLM client, server imports, error handling

Team 1: Critical Bug Fix Squad

Fixed:
- #1: LLM Client async methods use undefined self.client
  Changed to self._async_client in utils/llm.py
- #3: Server Upload endpoint missing UploadFile/File imports
  Added to server.py imports
- #17: LangGraph workflow error recovery
  - Replaced fake fallback with proper error raising
  - Enabled MemorySaver checkpointing for state persistence
  - Added traceback printing for debugging

opus_orchestrator/server.py

@@ -7,7 +7,7 @@ import os
 from typing import Any, Optional
 from contextlib import asynccontextmanager
 
-from fastapi import FastAPI, HTTPException, BackgroundTasks
+from fastapi import FastAPI, HTTPException, BackgroundTasks, UploadFile, File
 from fastapi.responses import JSONResponse, RedirectResponse
 from pydantic import BaseModel, Field
 from dotenv import load_dotenv