Add server-side per-problem video recording that allows teachers to click
on completed problems in the observer progress indicator to watch recorded
videos of each problem attempt.
Key changes:
- New vision_problem_videos schema and migration for per-problem videos
- VisionRecorder refactored for per-problem frame management
- Socket server triggers ffmpeg encoding on problem transitions
- API routes for streaming videos and listing available recordings
- SessionObserverModal enables browse mode on progress indicator
- ProblemVideoPlayer component for viewing past problem recordings
Each problem gets its own MP4 video encoded incrementally as it completes,
making playback immediately available for recently completed problems.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Thomas Hallock