Refactor importer segment writing
Extract a shared write_segment() helper from the segment-writing path and refactor the text importer to use it without changing behavior.
Rewrite the ChatGPT importer to emit segment-aligned imported_audio.jsonl output using 5-minute windows and extend ImportResult with optional segment metadata.
Update the CLI to prepare file-import directories and handle segment-producing file importers, including stream marker and segment tracking support.