(feature) Show next scheduled run and backup run type in the UI

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.

src/pobsync_backend/stats_summary.py

@@ -82,6 +82,7 @@ def _run_summary(run: BackupRun) -> dict[str, Any]:
     return {
         "id": run.id,
         "host": run.host.host,
+        "run_type": run.run_type,
         "started_at": run.started_at,
         "ended_at": run.ended_at,
         "snapshot": run.snapshot,