fix(6): update standalone start_server.sh to use Python virtual environment

- Create .venv automatically if missing
- Activate virtual environment before pip install
- Fixes PEP 668 externally-managed-environment error on Python 3.12
- Virtual environment isolated from system Python packages

start_server.sh

@@ -104,6 +104,16 @@ if [[ ! -f "requirements.txt" ]]; then
   log_error "backend/requirements.txt not found"
 fi
 
+# Create and use Python virtual environment
+VENV_DIR="../.venv"
+if [[ ! -d "$VENV_DIR" ]]; then
+  log_info "  Creating Python virtual environment..."
+  python3 -m venv "$VENV_DIR" || log_error "Failed to create virtual environment"
+fi
+
+# Activate virtual environment
+source "$VENV_DIR/bin/activate"
+
 # Install dependencies if needed
 if ! python3 -c "import fastapi" &> /dev/null; then
   log_info "  Installing Python dependencies..."