feat(app): node-override secrets UX + supabase scaffold refresh

- Secrets page: 'Node overrides' section (per-node values that shadow a shared
  secret on that node) — list/reveal/edit/delete + add; shared secrets show an
  'override: <node>' badge. Authority-only writes.
- API: GET /secrets/overrides, GET/PUT/DELETE /secrets/overrides/{node}/{name};
  OpenBaoBackend.list_node_overrides enumerates castle/nodes/*
- supabase scaffold: drop stale 'cat ~/.castle/secrets/...' + 'castle deploy',
  point at the dashboard Secrets page / castle apply
- tests: file-backend override endpoints (empty/400/404)

Verified live: primer's postgres override discoverable + full CRUD round-trip.

castle-api/tests/test_secrets_overrides.py

@@ -0,0 +1,24 @@
+"""HTTP-layer tests for the node-override endpoints (file-backend paths).
+
+The live OpenBao override round-trip is verified manually; here we pin the
+file-backend behavior (overrides are an OpenBao-only feature).
+"""
+
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+
+
+def test_list_overrides_empty_on_file_backend(client: TestClient) -> None:
+    r = client.get("/secrets/overrides")
+    assert r.status_code == 200
+    assert r.json() == {"overrides": {}}
+
+
+def test_set_override_rejected_on_file_backend(client: TestClient) -> None:
+    r = client.put("/secrets/overrides/primer/POSTGRES_PASSWORD", json={"value": "x"})
+    assert r.status_code == 400
+
+
+def test_get_missing_override_is_404(client: TestClient) -> None:
+    assert client.get("/secrets/overrides/primer/NOPE").status_code == 404