chore(retrieval): swap rank-bm25 for bm25s in BEIR harness, drop dead prod dep

[sanjeevafk]

rank-bm25 had zero production usage (prod lexical is Postgres ts_rank) and is unmaintained since 2021. This swaps the BEIR harness to maintained bm25s and removes the dead prod dep.

Parity gate (scifact 300/25, before -> after):

• Lexical NDCG 0.8025 -> 0.7905, recall unchanged at 0.84
• Top-1 agreement 23/25, mean top-10 overlap 90.8% (delta is idf-smoothing minutiae, no systematic shift)
• Lexical p50 1.34ms -> 0.11ms (12x, compounds at Tier-1 scale)

Changes:

• scripts/benchmark_beir.py: bm25s.BM25().index() on identical .lower().split() tokens (no stemming, comparability preserved); get_scores call site API-identical
• pyproject.toml: rank-bm25 removed from prod deps, bm25s>=0.3.0 added to dev extras
• api/requirements.txt: rank-bm25 removed (slims prod image)

Note: prior results.json lexical columns are now non-comparable history; future runs anchor to 0.7905.

Verification: 177/177 pytest pass, ruff + compileall clean.