PostgreSQL sink: add a pg.Pool error handler for resilience

[dcoric]

Follow-up to #1497 / #1532.

ensurePool() in src/db/postgres/helper.ts creates new Pool({ connectionString }) with no 'error' listener. Per the node-postgres docs, an idle client that emits 'error' with no listener throws an uncaught exception and can crash the process.

Scope

Attach pool.on('error', …) to log and swallow idle-client errors so a transient backend hiccup doesn't take the proxy down.