feat(config,caddy): admin spa under /admin and memory_root for the api frontend

config.py

@@ -245,6 +245,7 @@ frontends: list[Frontend] = [
         public_base_url=_public("/api"),
         master_agent="beaver-dispatcher",
         branch_agent="beaver-dispatcher",
+        memory_root=BEAVER,
     ),
     MarkdownFrontend(
         host="0.0.0.0",