Optimize memory for 512MB free tier: tune ONNX session options and reduce TF-IDF features (#11)

[SparshM8]

This PR implements memory optimizations for the 512MB free-tier host:

1. Tuned ONNX Runtime SessionOptions to disable memory pre-allocation and aggressive optimizations.
2. Reduced TF-IDF vectorizer max_features from 20,000 to 10,000 to shrink the similarity index footprint.
3. Added session config entries to use the device allocator for initializers.

These changes are expected to reduce the runtime memory footprint by 50-80 MB, providing safer headroom for concurrent requests.