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/server/test_semantic_consent_narrowing.py

@@ -0,0 +1,37 @@
+"""Unit tests for the search-side consent narrowing in nc_semantic_search.
+
+The narrowing logic (intersect requested doc_types with the admin allow-set,
+or restrict to the allow-set when none requested) is extracted into
+``_consent_narrowed_doc_types`` so it can be tested without exercising the full
+search path. ``allowed is None`` (no restriction / fail-open) is handled by the
+caller skipping this helper entirely.
+"""
+
+from __future__ import annotations
+
+from nextcloud_mcp_server.server.semantic import _consent_narrowed_doc_types
+
+
+def test_none_request_restricts_to_allow_set():
+    # No explicit doc_types -> search exactly the allowed set (sorted).
+    assert _consent_narrowed_doc_types(None, frozenset({"file", "note"})) == [
+        "file",
+        "note",
+    ]
+
+
+def test_request_intersected_with_allow_set_preserves_order():
+    result = _consent_narrowed_doc_types(
+        ["deck_card", "note", "file"], frozenset({"note", "file"})
+    )
+    assert result == ["note", "file"]
+
+
+def test_disjoint_request_yields_empty():
+    # Caller short-circuits to an empty response on [].
+    assert _consent_narrowed_doc_types(["deck_card"], frozenset({"note"})) == []
+
+
+def test_empty_allow_set_blocks_all():
+    assert _consent_narrowed_doc_types(None, frozenset()) == []
+    assert _consent_narrowed_doc_types(["note"], frozenset()) == []