feat: Implement two-pass encoding for better quality/size balance

[wmehanna]

Context\nTwo-pass encoding can significantly improve quality-to-size ratio by analyzing the video content in the first pass before encoding in the second pass.\n\n## Research\n- FFmpeg supports two-pass encoding via and \n- Tdarr and FileFlows use this for quality-critical workloads\n- Particularly useful for variable bitrate (VBR) optimization\n\n## Proposed Implementation\n1. Add two-pass encoding option to policy/encoding settings\n2. Implement first pass (analyze) - no output, generates stats file\n3. Implement second pass (encode) - uses stats file for better allocation\n4. Track pass completion in job metadata\n\n## Priority\nMedium - improves encoding quality for users willing to trade encode time