fix(vector): address PR review round 4 — backfill resilience + degraded-mode docs

- Remove three stale `# Use numeric file ID` / `# Pass file path` comments
  in scanner.py. file_id is already normalized to str() above each call
  site, so the inline comments mislead readers.
- Wrap `_backfill_doc_id_to_string` scroll loop + sentinel upsert in
  try/except Exception. The qdrant_client singleton is assigned before
  this migration runs, so a transient scroll failure was leaving the
  process holding a usable client with int payloads permanently
  unbackfilled until the next restart. Catch broadly, log ERROR with
  exc_info, and return without writing the sentinel — next process
  restart retries from scratch.
- Note `:memory:` mode behavior near the sentinel constants so future
  readers don't read the every-start scroll as a bug.
- Document the two degraded-migration ERROR log signals in
  docs/configuration.md so operators know when a clean restart is
  required to recover indexing.
- Add unit test asserting scroll-time exceptions are logged and swallowed
  without writing the sentinel.

Closes round-4 review feedback on PR #773.

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

tests/unit/vector/test_qdrant_client.py

@@ -387,3 +387,34 @@ async def test_backfill_handles_payload_with_explicit_none_doc_id(mocker):
         points=[2],
         wait=True,
     )
+
+
+@pytest.mark.unit
+async def test_backfill_logs_and_returns_when_scroll_raises(mocker, caplog):
+    """A scroll-time exception is logged and swallowed; sentinel is not written.
+
+    The singleton client in get_qdrant_client is already assigned by the
+    time _backfill_doc_id_to_string runs, so re-raising here would leave
+    the process holding a usable client with the migration silently
+    skipped on every subsequent call. Catching, logging, and returning
+    without writing the sentinel preserves retry-on-next-restart behavior.
+    """
+    client = mocker.AsyncMock()
+    client.retrieve.return_value = []  # No sentinel — backfill must run
+    client.scroll.side_effect = RuntimeError("boom")
+
+    with caplog.at_level("ERROR", logger="nextcloud_mcp_server.vector.qdrant_client"):
+        await _backfill_doc_id_to_string(
+            client, "test-collection", _backfill_dimension()
+        )
+
+    # No sentinel written — next process restart will retry from scratch.
+    client.upsert.assert_not_awaited()
+    client.set_payload.assert_not_awaited()
+    errors = [r for r in caplog.records if r.levelname == "ERROR"]
+    assert len(errors) == 1
+    assert "doc_id backfill failed" in errors[0].getMessage()
+    assert "test-collection" in errors[0].getMessage()
+    # exc_info=True attaches the original exception to the log record.
+    assert errors[0].exc_info is not None
+    assert errors[0].exc_info[0] is RuntimeError