(feature) Link rsync logs from backup run detail

Record the final rsync log path for successful real backup runs, matching
the existing dry-run and failure result payloads.

Add a staff-only run log endpoint and surface the link on run detail pages,
including fallback log discovery for older runs based on snapshot_path.

Cover direct log links and inferred scheduled backup logs with view tests.

src/pobsync_server/urls.py

@@ -27,6 +27,7 @@ urlpatterns = [
     path("hosts/<str:host>/retention-plan/", views.host_retention_plan, name="host_retention_plan"),
     path("hosts/<str:host>/schedule/", views.edit_host_schedule, name="edit_host_schedule"),
     path("runs/<int:run_id>/", views.run_detail, name="run_detail"),
+    path("runs/<int:run_id>/rsync-log/", views.run_rsync_log, name="run_rsync_log"),
     path("runs/<int:run_id>/cancel/", views.cancel_run, name="cancel_run"),
     path("snapshots/<int:snapshot_id>/", views.snapshot_detail, name="snapshot_detail"),
     path("api/", api.api_index),