fix(vector): address PR review round 3 — sentinel guard, skip indexed fields, narrow types

- Add a fixed-UUID sentinel point written after a successful doc_id
  backfill so subsequent restarts retrieve it and short-circuit the
  O(N) scroll. Sentinel has no user_id/doc_id/doc_type payload so
  production search filters never see it.
- Pre-fetch payload_schema in _ensure_keyword_payload_indexes and
  silently skip fields that are already indexed; the "Created KEYWORD
  payload index" INFO log fires only on actual creation.
- Narrow stale `int | str` doc_id annotations to `str` across
  search/verification.py (BatchVerifier return type, per-verifier
  accessible sets, by_type / accessible_by_type / inaccessible
  collections); drop the now-redundant `type(d).__name__` prefix in
  the dropped-docs log.
- Align the backfill log message with the PR description's
  "Running doc_id backfill" promise; add a caller cross-reference to
  the wait=True comment.
- Fix _get_file_path_from_qdrant docstring (file_id is str, not numeric).
- Convert legacy `id=1` to `id="1"` in test_search_result.py to match
  the SearchResult.id: str annotation.

Three new unit tests cover sentinel-found, sentinel-written, and
skip-existing-index branches; existing backfill tests pass dimension
and explicit retrieve.return_value=[] for the no-sentinel path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/search/context.py

@@ -151,7 +151,8 @@ async def _get_file_path_from_qdrant(
 
     Args:
         user_id: User ID who owns the file
-        file_id: Numeric file ID
+        file_id: Stringified file ID (Qdrant payload value, post-doc_id
+            normalization — see vector/qdrant_client.py)
         chunk_start: Character offset where chunk starts
         chunk_end: Character offset where chunk ends