- 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
- Add VideoProcessor service to PostHandler for frame-based video moderation
- Implement multi-frame extraction and Azure OpenAI Vision analysis for video content
- Enhance VideoStitchingService with filters, speed control, and text overlays
- Add image upload dialogs for group avatar and banner in GroupCreationModal
- Implement navigation placeholders for mentions, hashtags, and URLs in sojornRichText
