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>

nextcloud_mcp_server/vector/processor.py

@@ -658,6 +658,23 @@ async def _index_document(
     indexed_at = int(time.time())
     points = []
 
+    # Surface deck card data quality issues at indexing time rather than
+    # only at verification time (where _verify_deck_cards falls through to
+    # legacy-data pass-through when board_id/stack_id are missing). This is
+    # logged once per document — not per chunk — to avoid log spam.
+    if doc_task.doc_type == "deck_card":
+        missing_deck_fields = [
+            field for field in ("board_id", "stack_id") if not file_metadata.get(field)
+        ]
+        if missing_deck_fields:
+            logger.warning(
+                "Indexing deck_card %s for user %s with missing metadata: %s; "
+                "verification will fall back to legacy-data pass-through",
+                doc_task.doc_id,
+                doc_task.user_id,
+                missing_deck_fields,
+            )
+
     for i, (chunk, dense_emb, sparse_emb) in enumerate(
         zip(chunks, dense_embeddings, sparse_embeddings)
     ):