S3 user-metadata values are sent as HTTP headers, which must be
US-ASCII. Notion page titles routinely contain em dashes and other
non-ASCII characters, causing ERR_INVALID_CHAR and a daily sync
failure. Sanitize the notion-title metadata to printable ASCII.
Also treat a Bedrock ConflictException from StartIngestionJob as
benign (an ingestion job is already running and will pick up the
freshly uploaded files) instead of failing the whole invocation.
