fix(vector): address PR review round 11 — broaden offset-skip gate, clarify ordering

- search/context.py: drop the doc_type=='file' guard on skip_offset_lookup
  so notes / deck cards / news items also bypass the unindexed offset
  fallback when chunk_index is available. Legacy chunk_index=None data
  still uses the offset path.
- vector/qdrant_client.py: clarify the backfill/_ensure_payload_indexes
  ordering invariant (backfill rewrites payload values only, never schema
  or indexes). Acknowledge OSS-vs-Cloud uncertainty in the 400-branch
  comment and the new-collection call-site comment.
- vector/scanner.py: hoist qdrant_client to function scope so the
  file-scroll block doesn't depend on a name bound inside the
  notes-scroll block.
- tests/unit/test_chunk_context_offset_gate.py: flip the note-with-
  chunk_index test to assert the offset fallback is skipped.

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

nextcloud_mcp_server/search/context.py

@@ -373,12 +373,15 @@ async def get_chunk_with_context(
         chunk_text = await _get_chunk_by_index_from_qdrant(
             user_id, doc_id, doc_type, chunk_index
         )
-    # Skip the offset fallback for files when the indexed chunk_index
-    # lookup already ran: chunk_start/end_offset aren't indexed in Qdrant
-    # Cloud strict mode, so the call returns 400 and surfaces a misleading
-    # logger.error. The file fast-fail below correctly handles the miss
-    # without it.
-    skip_offset_lookup = chunk_index is not None and doc_type == "file"
+    # When chunk_index is available, treat the indexed lookup as canonical
+    # for every doc_type. A miss means the chunk is genuinely absent, not
+    # "fall back to the unindexed slow path". chunk_start/end_offset aren't
+    # in _PAYLOAD_INDEX_FIELDS, so the offset filter 400s in Qdrant Cloud
+    # strict mode and surfaces a misleading logger.error. Legacy data
+    # without chunk_index (pre-cbcoutinho/astrolabe#75) still hits the
+    # offset path and degrades to a None chunk with a WARNING; that's the
+    # same behavior get_chunk_bbox_and_page_from_qdrant already documents.
+    skip_offset_lookup = chunk_index is not None
     if chunk_text is None and not skip_offset_lookup:
         chunk_text = await _get_chunk_from_qdrant(
             user_id, doc_id, doc_type, chunk_start, chunk_end