search.py: add VOYAGE_API_KEY guard at startup

query.py validates VOYAGE_API_KEY at startup but search.py doesn't. Without it, users get a confusing NoneType error instead of a clear message.

**Fix:**
```python
if not VOYAGE_API_KEY:
    print("[ERROR] VOYAGE_API_KEY not set in .env"); sys.exit(1)
```

Add this after `load_dotenv()` in search.py.