BLAST database recommendation endpoint

Motivation

New users often do not know which database to point a search at — core_nt vs nt, nr vs refseq_protein vs swissprot. NCBI Web BLAST nudges the user toward a sensible default based on what they are trying to do. A small, versioned rule table can give the same nudge with no Azure calls and a stable, testable contract.

User-facing change

• New backend route GET /api/blast/databases/recommend with query params molecule (dna|protein), program, goal (identify|highly_similar|transcripts|genomes|well_characterized|comprehensive, default identify), and optional taxon.
• Returns a recommended database plus an alternative, each with a short rationale, and the ruleset version (2026-06-01). When a taxon is supplied the response notes that taxonomy is applied as a -taxids filter, not a database switch.

This is the backend contract for an upcoming submit-form database hint; the endpoint is live and tested now.

API / IaC diff summary

• New service api/services/blast/db_recommendation.py — recommend_database(...) → Recommendation over a static _RULES table (pure function, no Azure SDK).
• New route blast_databases_recommend in api/routes/blast/databases.py. Distinct from the existing /databases/{db_name}/oracle route, which is a shard-order pointer optimization, not a database selection helper.
• No infra change.

Validation evidence

• uv run pytest -q api/tests/test_blast_db_recommendation.py — 8 passed (per-goal rules, program→molecule inference, taxon note, HTTP route).
• uv run ruff check api — all checks passed.