Deep audit fixes: Enhanced FFmpeg transcoding capabilities

[alokemajumder]

Summary

This PR addresses gaps found during the deep FFmpeg integration audit to ensure the API can fully utilize FFmpeg's capabilities.

FFmpegWrapper Fixes

• Updated validate_operations method with all new operation types (was missing scale, crop, rotate, flip, audio, subtitle, concat)
• Fixed trim handler to support both start/start_time and end/end_time naming conventions
• Allow empty operations list for simple format conversion

Enhanced Transcode Operation

Feature	Description
Profile	H.264/H.265 profiles (baseline, main, high, high10, high422, high444)
Pixel Format	yuv420p, yuv422p, yuv444p, 10-bit variants
Hardware Accel	User-selectable: auto, none, nvenc, qsv, vaapi, videotoolbox
VBV Buffer	max_bitrate, buffer_size for streaming compliance
GOP Size	Keyframe interval control (1-600)
B-Frames	B-frame count (0-16)
Audio	Sample rate, channels control
Faststart	Auto-enabled for web streaming
Two-Pass	Two-pass encoding flag

New Codec Support

• Video: prores, dnxhd (professional codecs)
• Audio: eac3, flac, pcm_s16le, pcm_s24le (lossless/uncompressed)

Example Payloads

Professional H.264 encode with VBV

{
  "input": "/storage/input/video.mp4",
  "output": "/storage/output/broadcast.mp4",
  "operations": [{
    "type": "transcode",
    "video_codec": "h264",
    "profile": "high",
    "preset": "slow",
    "video_bitrate": "8M",
    "max_bitrate": "10M",
    "buffer_size": "20M",
    "gop_size": 48,
    "b_frames": 2,
    "pixel_format": "yuv420p"
  }]
}

Hardware-accelerated encode

{
  "input": "/storage/input/video.mp4",
  "output": "/storage/output/fast.mp4",
  "operations": [{
    "type": "transcode",
    "video_codec": "h264",
    "hardware_acceleration": "nvenc",
    "crf": 23
  }]
}

Test plan

• Python syntax validation passes
• Unit tests for new validators
• Integration test with hardware acceleration detection