(ui) Make retention planning warnings explicit

Show keep/delete reasons in the retention plan, surface scheduled prune
limit warnings, and explain base snapshot protection before retention is
applied.

Also surface incomplete snapshots from the retention views without deleting
them automatically, so interrupted backups are visible on the dashboard,
host detail, and retention plan.

src/pobsync_backend/tests/test_sql_retention.py

@@ -32,7 +32,10 @@ class SqlRetentionTests(TestCase):
 
         self.assertEqual(plan["source"], "sql")
         self.assertEqual(plan["keep"], [new.dirname])
+        self.assertEqual([item["dirname"] for item in plan["keep_items"]], [new.dirname])
         self.assertEqual([item["dirname"] for item in plan["delete"]], [old.dirname])
+        self.assertEqual(plan["delete"][0]["reason"], "outside retention policy")
+        self.assertEqual(plan["incomplete"], [])
 
     def test_plan_can_protect_base_snapshot_from_sql_relation(self) -> None:
         host = HostConfig.objects.create(