refactor(search): address PR #750 round 6 review feedback

Closes out the remaining nits flagged in the round-6 review.

Critical:
- _verify_files contract comment now enumerates all None-return cases
  (404 + malformed PROPFIND XML) and documents the false-eviction
  trade-off; self-healing via re-indexing recovers
- int(r.id) cast at the SemanticSearchResult boundary now raises a
  TypeError with explicit doc_type/value context instead of bubbling
  up as an opaque "Search failed: ..." McpError

Design observations:
- nc_semantic_search_answer docstring documents the per-note
  round-trip cost from the post-verification race guard
- News verification latency hint added to configuration.md
- SemanticSearchResponse exposes verified_count + dropped_count so
  short result pages on high-ghost-density indexes are
  distinguishable from genuine scarcity. verify_search_results now
  returns (kept, dropped_count); production caller and tests updated

Minor:
- Comment clarifies the .get() fallback in verify_search_results is
  defensive only (run_verifier always populates the entry)
- Eviction task-group guard narrowed from except Exception to
  except RuntimeError (the only documented failure mode of
  TaskGroup.start_soon on a closed group)
- Indexer logs a warning when a deck_card task is missing
  board_id/stack_id, surfacing data-quality issues at index time
  rather than at verification time
- New unit test covers the news verifier's non-numeric-id fail-open
  path (one bad doc_id keeps the entire batch)

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

docs/configuration.md

@@ -498,9 +498,12 @@ aware of:
   verifier issues a single `news.get_items(batch_size=-1, get_read=True)` call
   per search that contains any news result, then intersects locally. The
   payload is **unbounded** — for users with very large feed backlogs this can
-  dominate verification latency. Disabling News in the indexer or running with
-  a smaller backlog mitigates this; per-item paginated verification is tracked
-  as a future improvement.
+  dominate verification latency. As a rough guide on a healthy LAN connection:
+  a typical purged backlog (1k–5k items) returns in ~200–500 ms; very large
+  backlogs (>20k items) can exceed 2 s and become the dominant cost of any
+  search that surfaces news results. Disabling News in the indexer or running
+  with a smaller backlog mitigates this; per-item paginated verification is
+  tracked as a future improvement.
 - **Eviction**: when verification finds a definitive miss (404 / 403), the
   corresponding Qdrant points are deleted in the background on a lifespan-owned
   task group — fire-and-forget, does **not** block the search response.