(feature) Capture structured backup statistics

Parse rsync --stats output into structured run metrics for file counts,
transferred bytes, literal data, matched data, speedup, and estimated
link-dest savings.

Store collected stats on backup run results and successful snapshot metadata,
including snapshot data usage and backup-root capacity details for future
dashboard graphs and disk-full projections.

Render the collected metrics on run and snapshot detail pages, with tests
covering parsing, metadata persistence, and UI output.

src/pobsync_backend/views.py

@@ -348,10 +348,12 @@ def queue_manual_backup(request, host: str):
 @staff_member_required
 def run_detail(request, run_id: int):
     run = get_object_or_404(BackupRun.objects.select_related("host", "snapshot"), id=run_id)
+    run_stats = run.result.get("stats") if isinstance(run.result, dict) else {}
     context = {
         "run": run,
         "can_cancel": run.status in {BackupRun.Status.QUEUED, BackupRun.Status.RUNNING},
         "requested": run.result.get("requested") if isinstance(run.result, dict) else {},
+        "stats": run_stats if isinstance(run_stats, dict) else {},
         "result_json": _pretty_json(run.result),
     }
     return render(request, "pobsync_backend/run_detail.html", context)
@@ -389,6 +391,7 @@ def snapshot_detail(request, snapshot_id: int):
     )
     context = {
         "snapshot": snapshot,
+        "stats": snapshot.metadata.get("stats") if isinstance(snapshot.metadata, dict) else {},
         "metadata_json": _pretty_json(snapshot.metadata),
         "backup_runs": snapshot.backup_runs.select_related("host").order_by("-created_at"),
         "derived_snapshots": snapshot.derived_snapshots.select_related("host").order_by("-started_at", "dirname"),