fix(db): cap postgres connection pools

[raymondjacobson]

Summary

• add a shared dbpool helper that applies conservative connection-pool defaults to long-lived Postgres pools
• cap core, eth, and mediorum pgx pools at 8 max conns by default while preserving explicit pgxpool DSN params
• reduce mediorum GORM/sql pool from 50 max open conns to a 20 max / 5 idle default with env overrides

Pool knobs

• OPENAUDIO_PGX_POOL_MAX_CONNS
• OPENAUDIO_PGX_POOL_MAX_CONN_IDLE_TIME
• OPENAUDIO_GORM_MAX_OPEN_CONNS
• OPENAUDIO_GORM_MAX_IDLE_CONNS
• OPENAUDIO_GORM_CONN_MAX_IDLE_TIME

Testing

• go test ./pkg/dbpool ./pkg/core ./pkg/eth
• go test -c ./pkg/mediorum/server -o /tmp/mediorum-server.test

Notes

• go test ./pkg/mediorum/server still requires its local Postgres harness on localhost:5454; without that harness it fails during TestMain database setup before package tests run.