fix(search): address PR #813 review (viz verify-on-read, owners cache, docs)

- viz_routes: run verify_search_results before returning results. After the
  accessible_owners expansion the viz can surface OTHER users' shared docs, so
  it must drop ones the caller can no longer access (revoked share) — same as
  the nc_semantic_search tool path. (Blocking review item.)
- access_filter: cache list_accessible_owners per user for 30s to keep the OCS
  shares round-trip off the search hot path (failures aren't cached); document
  the single-page OCS limitation; add a clear_accessible_owners_cache() test
  helper. Comment the empty-accessible_owners MatchAny([]) edge case.
- verification: comment why cross-user eviction is a deliberate no-op (eviction
  is scoped to the querying user's id, so a recipient's revoked access never
  deletes the owner's points; the recipient self-heals via accessible_owners).
- algorithms: declare SearchResult.original_score (set by the viz route) so the
  now-precisely-typed result list type-checks.
- tests: cross-user eviction-no-op safety test; autouse owners-cache reset in
  the access_filter + shared-search tests; replace async-no-await qdrant fakes
  with AsyncMock (clears SonarCloud S7503).

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

nextcloud_mcp_server/search/algorithms.py

@@ -168,6 +168,9 @@ class SearchResult:
     chunk_index: int = 0
     total_chunks: int = 1
     point_id: str | None = None
+    # Pre-normalization score, set by the visualization route before it rescales
+    # ``score`` to [0, 1] for visual encoding (see auth/viz_routes.py).
+    original_score: float | None = None
 
     def __post_init__(self):
         """Validate score is non-negative.