0066 — MaxConns is set explicitly on the datastore pool

Context. pgxpool’s default pool_max_conns is max(4, nCPU) — 10 on this box. With one connection per concurrent per-message commit, any worker count above ~10 silently queues on the pool rather than on Postgres.

Decision. Expose and set MaxConns (pool_max_conns) on the datastore so the pool scales with the configured worker count.

Consequences. Removes a fake ceiling that would otherwise masquerade as a database limit in throughput measurements; worker-count sweeps above 10 become meaningful. The pool size becomes a knob that must track worker configuration.