build: add faiss dependency for two-tower retrieval

[saurabhhhcodes]

Summary

Fixes #1630.

What changed

• Added faiss-cpu>=1.8.0 to requirements.txt so the two-tower retrieval engine can import faiss during test collection and runtime setup.

Validation

• git diff --check
• Verified the dependency entry is present in requirements.txt

Notes

This branch is stacked on top of the recommendation route cleanup branch so the dependency lane can keep moving without waiting on that review to land.

[github-actions]

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Labels added: gssoc:approved | mentor:leonagoel | status:review-needed

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