Decouple Media Upload from Full AI Analysis in Smart Clip; Support On-Demand Timeline Batching

Production editing sessions frequently import hundreds of raw clips while only ~20% make the rough-cut timeline. Running full multimodal frame extraction and transcription on every upload burns token quota and delays workspace readiness. Proposal: 1. Extract only lightweight metadata and proxy thumbnails on upload, keeping auto-analyze off by default (with an opt-in toggle); 2. Add an on-demand "Batch AI Analyze" action on selected timeline clips with bounded concurrency and per-clip progress.