Bring the Server (Postgres) workflow in line with Server (MariaDB) internals while
keeping it opt-in for now: pull_request runs still require the 'postgres' label, but the
job now uses the full 4-container matrix (was 1), adds the nightly schedule /
workflow_dispatch / repository_dispatch triggers (which always run), and uploads
coverage. Builds ERPNext against frappe `develop` (PostgreSQL query-builder/ORM support
is merged there), so no fork override is needed.
The ERPNext server suite now passes on PostgreSQL and MariaDB from a single codebase;
flipping this to run on every PR / become a required check is a later, separate step.
