The sub-issue progress indicator (e.g. "0/2") was undercounting because
it was computed from the client-side issue list, which only loads the
first 50 done issues. Sub-issues marked as done beyond that page were
excluded from both the total and done counts.
Added a dedicated backend endpoint (GET /api/issues/child-progress) that
aggregates child issue counts directly from the database, ensuring
accurate totals regardless of client-side pagination or filtering.
Fixes MUL-702