Enhance FFmpeg operations for full transcoding capabilities

[alokemajumder]

Summary

This PR significantly enhances the FFmpeg integration to expose more of FFmpeg's capabilities through the API.

New Operations Added

• scale - Resolution scaling with algorithm selection (lanczos, bicubic, etc.)
• crop - Video cropping with FFmpeg expression support
• rotate - Video rotation (90°, 180°, 270°, or arbitrary angles)
• flip - Horizontal/vertical flip
• audio - Audio processing (volume, normalization, sample rate, channels)
• subtitle - Subtitle overlay (SRT, ASS, VTT support)
• concat - Video concatenation

Enhanced Filters (50+ FFmpeg filters now available)

• Video Transform: scale, crop, overlay, pad, transpose, hflip, vflip, rotate
• Color/Quality: eq, hqdn3d, unsharp, colorchannelmixer, lut3d, curves
• Deinterlacing: yadif, bwdif, w3fdif, nnedi
• Audio: volume, loudnorm, dynaudnorm, atempo, amerge, amix
• Effects: fade, afade, drawtext, boxblur, gblur
• Stabilization: vidstabdetect, vidstabtransform, deshake
• HDR: zscale, tonemap, colorspace

API Improvements

• Support both flat and nested params structure for flexibility
• Allow empty operations list for simple format conversion
• Map 'stream' and 'streaming' operation types consistently

Test plan

• Python syntax validation passes
• Unit tests for new validators
• Integration test with sample video files

Example payloads

Scale video to 1080p

{
  "input": "/storage/input/video.mp4",
  "output": "/storage/output/video_1080p.mp4",
  "operations": [
    {"type": "scale", "width": 1920, "height": 1080, "algorithm": "lanczos"}
  ]
}

Audio normalization

{
  "input": "/storage/input/video.mp4",
  "output": "/storage/output/video_normalized.mp4",
  "operations": [
    {"type": "audio", "normalize": true, "normalize_type": "loudnorm"}
  ]
}

Crop and rotate

{
  "input": "/storage/input/video.mp4",
  "output": "/storage/output/video_processed.mp4",
  "operations": [
    {"type": "crop", "width": 1280, "height": 720, "x": 100, "y": 50},
    {"type": "rotate", "angle": 90}
  ]
}