Make DB pool size configurable; warn when queue concurrency outruns it

Commit notes

- DB_POOL_MAX env var (default 20) controls the postgres-js pool; read directly from process.env so scripts can import db with partial env. - Worker logs a boot-time warning when the combined queue concurrency exceeds the pool size instead of silently queueing queries under load. - Fix env.example DATABASE_URL user to match docker-compose (beatstep, not the stale samplestep), and document DB_POOL_MAX.

[private session redacted]