(feature) Add Django retention apply flow

Expose retention apply from the host retention plan page so planned
snapshot deletions can be executed from the Django UI.

The form requires explicit host confirmation, carries through the
selected retention kind and base-protection setting, and uses max_delete
as a deletion guard. The view delegates to the SQL retention apply
service and reports predictable pobsync errors back through Django
messages instead of surfacing a server error.

Add view coverage for confirmed deletion, invalid confirmation, and
POST-only enforcement.

src/pobsync_server/urls.py

@@ -14,6 +14,7 @@ urlpatterns = [
     path("hosts/<str:host>/config/", views.edit_host_config, name="edit_host_config"),
     path("hosts/<str:host>/queue-backup/", views.queue_manual_backup, name="queue_manual_backup"),
     path("hosts/<str:host>/discover-snapshots/", views.discover_host_snapshots, name="discover_host_snapshots"),
+    path("hosts/<str:host>/retention-apply/", views.apply_host_retention, name="apply_host_retention"),
     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"),