fix: use uvicorn directly without reload mode for backend startup

- Fix backend process dying immediately after startup
- Change from 'python -m app.main' to 'uvicorn app.main:app'
- Remove reload mode for stable background operation
- Resolve 'connect ECONNREFUSED 127.0.0.1:8000' error

Problem:
- Backend used reload=settings.DEBUG mode
- reload mode conflicts with nohup background running
- Process management became unstable
- Backend service stopped immediately after startup

Solution:
- Use uvicorn command directly without reload
- Single process mode, stable background operation
- Suitable for production environment

Files:
- start.sh: updated backend startup command
- docs/BACKEND_FIX.md: fix documentation

start.sh

@@ -68,7 +68,7 @@ fi
 echo -e "${GREEN}启动后端服务...${NC}"
 cd "$BACKEND_DIR"
 source venv/bin/activate
-nohup python -m app.main >> "$BACKEND_LOG" 2>&1 &
+nohup uvicorn app.main:app --host 0.0.0.0 --port 8000 >> "$BACKEND_LOG" 2>&1 &
 BACKEND_PID=$!
 echo "$BACKEND_PID" >> "$PID_FILE"
 echo -e "  PID: $BACKEND_PID"