Each BqAccess.duckdb_session() acquire previously created a fresh
in-memory DuckDB conn and ran INSTALL bigquery; LOAD bigquery;
CREATE SECRET; ATTACH on it -- costing ~0.5 s per request even before
any BQ work. Add a process-local pool (deque + lock) of pre-warmed
sessions; acquire reuses a warm entry when available, refreshing the
auth SECRET so a long-lived pool entry doesn't keep a stale GCE
metadata token past its TTL. Liveness probe (cheap SELECT 1) drops
broken entries before handing them to callers.
On exception inside the with-block the conn is closed instead of
returned to pool (session may carry dirty state). Pool size is
data_source.bigquery.session_pool_size (default 4; sentinel 0
disables pooling). Process-cached, not fork-safe (single uvicorn
worker is the supported deployment shape per CLAUDE.md).
All call sites get faster automatically: /api/query, /api/v2/{scan,
sample,schema}, materialize, the orchestrator's remote-attach, and
the BQ dry-run cap-guard.
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