feat: add pre-built demo FAISS index (nyc_taxi_2023, 10K vectors, 589 KB)

Ships a reproducible IVF32,PQ8×8 index built from 10,000 real NYC Yellow
Taxi trips (January 2023).  Clone → ./scripts/run_demo.sh is now fully
self-contained: the index is already present so the embedding step only
runs on the very first checkout without the committed file.

Verified end-to-end:
  - cold search ~8 s (CPU sentence-transformer warmup)
  - warm cache hit 73 ms (Redis)
  - cacheHit: true on repeated query

Author: ritunjaym