fix(vector-sync): address round-2 review — one-shot backstop, helper, caps

- scanner: gate the consent backstop with a per-(user,doc_type) one-shot
  marker so a standing admin-disable doesn't re-enqueue idempotent deletes
  every scan tick; the marker clears when the type is re-enabled. Derive
  _TEXT_BACKSTOP_DOC_TYPES from INDEXED_DOC_TYPES so new indexed types are
  covered automatically
- semantic: extract _consent_narrowed_doc_types so the search-side narrowing
  is unit-testable; add tests for restrict/intersect/disjoint/empty
- purge route: cap doc_types length (abuse guard) -> 400
- tests: one-shot + re-enable backstop, too-many-doc_types 400

Deferred (noted on PR): per-document allowed_doc_types call is cache-hot;
purge "last error wins" — both logged. SonarCloud broad-except hotspots are
deliberate (noqa BLE001), reviewable in the UI.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_vector_sync_purge_route.py

@@ -151,6 +151,20 @@ def test_bad_request_when_body_not_object(mocker):
     purge.assert_not_called()
 
 
+def test_bad_request_when_too_many_doc_types(mocker):
+    _patch_token(mocker)
+    purge = _patch_purge(mocker)
+
+    client = TestClient(_build_app())
+    resp = client.post(
+        "/api/v1/vector-sync/purge",
+        json={"doc_types": [f"t{i}" for i in range(65)]},
+    )
+
+    assert resp.status_code == 400
+    purge.assert_not_called()
+
+
 def test_provisioning_required_returns_428(mocker):
     _patch_token(mocker, "admin")
     mocker.patch(