(feature) Add cancellable backup runs and clearer dry-run logs

Add a cancel action for queued and running backup runs. Queued runs are
cancelled immediately, while running runs are marked for cancellation and
the worker terminates the active rsync process group.

Make dry-run log paths run-specific and add a defensive default dry-run
timeout so stuck dry-runs do not remain running indefinitely.

Remove rsync exit codes from run overview tables while keeping detailed
diagnostics available on the run detail payload.

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>/cancel/", views.cancel_run, name="cancel_run"),
     path("snapshots/<int:snapshot_id>/", views.snapshot_detail, name="snapshot_detail"),
     path("api/", api.api_index),
     path("api/status/", api.status),