feat(similarity): add precomputed similarity cache, fix fusion N+1, add 429 retry
- Add similarity/ package (compute.py, lookup.py) with numpy-based
pairwise cosine similarity and cached lookup
- database.py: create embeddings + similarity_cache tables in _init_database(),
add store_similarity_batch/get_cached_similarities/clear_similarity_cache helpers
- pipeline/fusion.py: replace N+1 per-motion embedding SELECT with single
bulk JOIN using DuckDB QUALIFY window function
- ai_provider.py: retry HTTP 429 with Retry-After header support
- migrations/2026-03-22-add-similarity-cache.sql: make executable
- Add tests for similarity compute, db helpers, and 429 retry (34 pass, 2 skip)
sgeboers
a78bee9b0a