The Worker now connects to Postgres exclusively through the HYPERDRIVE
binding. Local dev uses the binding's localConnectionString (committed in
wrangler.jsonc, pointing at the throwaway Docker Postgres from
docs/LOCAL_POSTGRES.md) instead of a POSTGRES_DATABASE_URL Worker var.
Closes the Codex finding about unpooled direct connections from deployed
Workers: the not-recommended direct-connection config is no longer possible.
POSTGRES_DATABASE_URL remains as a Node-side env var for drizzle-kit and
the D1->Postgres migration script only.
