fix(chunk-context): address PR #767 round-3 review — gate readability + legacy-fallback comment

- search/context.py: rename triple-negation gate condition to
  `skip_offset_lookup` named boolean for readability; convert new
  logger.warning to lazy %-style per repo convention.
- api/visualization.py, auth/viz_routes.py: add comment on the offset-only
  Qdrant scroll branch noting it is a legacy path for pre-astrolabe#75
  clients and degrades gracefully on Qdrant Cloud strict mode.

Reviewer item #2 (extracting the duplicated scroll block into a shared
helper) deferred to a follow-up issue per the reviewer's "not blocking"
framing.

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

nextcloud_mcp_server/api/visualization.py

@@ -601,6 +601,12 @@ async def get_chunk_context(request: Request) -> JSONResponse:
                         with_payload=["chunk_bbox", "page_number"],
                     )
                 else:
+                    # Legacy fallback for clients that don't send chunk_index
+                    # (pre-cbcoutinho/astrolabe#75). chunk_start/end_offset
+                    # aren't indexed in Qdrant Cloud strict mode, so this
+                    # call may fail with HTTP 400 there; the outer except
+                    # logs a warning and the response degrades gracefully
+                    # (no chunk_bbox).
                     points_response = await qdrant_client.scroll(
                         collection_name=settings.get_collection_name(),
                         scroll_filter=Filter(