(release) Add explicit incomplete snapshot cleanup

Add a dedicated cleanup path for incomplete snapshots instead of letting
retention prune them implicitly. The retention plan now exposes a guarded
form that requires host and delete-count confirmation before removing
.incomplete snapshot directories and their SQL records.

Keep scheduled/manual retention behavior unchanged, add path safety checks,
and cover cleanup success, confirmation failures, max-delete limits, and
unexpected paths in tests.

Refs #10

src/pobsync_server/urls.py

@@ -27,6 +27,11 @@ urlpatterns = [
     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>/incomplete-cleanup/",
+        views.cleanup_host_incomplete_snapshots,
+        name="cleanup_host_incomplete_snapshots",
+    ),
     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"),