Update the scheduler to reflect the actual scheduled BackupRun status after
a run completes, so prune warnings are shown as schedule warnings instead
of being reported as successful schedule executions.
Add a scheduler helper that calculates the next due time for a cron-style
schedule expression and surface that value on the dashboard and host detail
pages.
Show the latest run type in host summaries and backup trend tables so
manual and scheduled backups are distinguishable in the Django UI.
Keep the calculation derived from existing ScheduleConfig data without
adding a migration.
Treat SQL-backed Django models as the source of truth for pobsync
configuration, exporting runtime YAML only as a compatibility layer for
the existing engine. Add a database-driven scheduler command, Docker
scheduler services, schedule run-state fields, and tests for scheduler,
config export, and retention behavior.
