fix(tasks): Move GAL backfills to long pool (#123072)

All group action log and PR-lifecycle backfill tasks now publish through
the `issues.long` namespace and long-running processing pool. This
includes the group, project, enrollment, coordinator, reset, and
PR-lifecycle stages so self-chained work remains on the same pool.

The `issues` namespace remains registered as an alias so activations
already queued there can still be consumed during rollout. Task names,
processing deadlines, and backfill behavior are unchanged.