#965 Reduce idle PostgreSQL connections: pool_min_size 1, pool_max_idle 90s

EARS: While a Highway service is idle, each engine pool shall hold at most 1 connection, and an idle connection above min shall be released within 90 s per release window. Requested by infra-manager (AgentBus 01M53W4ZRSP6RE3HEGX8J9FDXD): vm-1 memory stall 02:20-02:40Z, 33 idle Highway backends ~1.1 GB. Config-only change (docker/config.ini [database]); rolled one container at a time; no build.