feat: Add pre-encoding quality prediction before transcoding

[wmehanna]

Problem\nCurrently BitBonsai only calculates quality metrics (VMAF/PSNR/SSIM) AFTER encoding completes. Users have no way to predict if the output will meet their quality threshold before investing time in encoding.\n\n## Solution\nAdd a pre-encoding quality prediction feature that uses lightweight analysis to estimate:\n- Predicted VMAF score\n- Whether output will pass the configured threshold\n- Recommended encoding settings for best quality/size tradeoff\n\n## Research Notes\n- Tdarr and FileFlows both offer similar "pre-check" features\n- FFmpeg can do quick analysis passes without full encoding\n- Could use bitrate analysis, resolution comparison, and source quality metrics\n\n## Acceptance Criteria\n- [ ] Quick analysis endpoint returns predicted quality score\n- [ ] UI shows "quality prediction" badge before encoding starts\n- [ ] Skip encoding automatically if prediction indicates threshold won't be met\n- [ ] Document the feature in CLAUDE.md