Fix: Import time module at top level for SSE events

The sse_event() helper function uses time.time() but time was only
imported inside the event_generator function, causing NameError.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/model_service.py

@@ -10,6 +10,7 @@ from pydantic import BaseModel
 import asyncio
 import json
 import os
+import time
 import torch
 from transformers import AutoModelForCausalLM, AutoTokenizer
 from typing import Optional, List, Dict, Any