- Go: Add overlay_json to Post model, CreatePost handler, and all post queries
- Flutter: Rename QuipTextOverlay → QuipOverlayItem with type enum (text/sticker)
- QuipOverlayItem: add id field, content replaces text, backward-compat alias
- quip_video_item: parse overlay_json and render non-interactive overlays in feed
- quip_upload_provider: accept overlayJson param and pass to publishPost
- api_service: add overlayJson param to publish
