Fix trim consistency: scene.json fps/duration use trim window (was 2x bug), video_source pinned to trim, frontend trim review pre-loads saved values

Step 1 full re-run @ 15fps with compound+fallback prompts and containment merge (55 clips)

Replace samples/ with 21-clip Step 1 test batch (no SAM3D, no 6DoF)

Refresh dataset: 48 trim-reviewed clips, manual SAM3 prompts, full descriptions in scene.json

Initial deploy