Add configurable fallback_rules system for selective local ffmpeg execution

[jordanhillis]

Summary

This update adds a new configuration option, fallback_rules, that lets users define when rffmpeg should automatically use local fallback_ffmpeg / fallback_ffprobe instead of remote hosts.

Configuration Example

rffmpeg:
  commands:
    ffmpeg: /usr/lib/jellyfin-ffmpeg/ffmpeg
    ffprobe: /usr/lib/jellyfin-ffmpeg/ffprobe
    fallback_ffmpeg: /usr/local/bin/ffmpeg
    fallback_ffprobe: /usr/local/bin/ffprobe

  fallback_rules:
    - match: ["-vf blackframe", "-f null"]    # Intro Skipper
    - match: ["-f chromaprint"]               # Chromaprint

Use Case Example

In Jellyfin, the Intro Skipper plugin runs short analysis commands like:

ffmpeg -hide_banner -loglevel warning -threads 0 -ss 0 -i /FILE/NAME/HERE.mkv -to 339.945 -ac 2 -f chromaprint -fp_format raw -

Previously, these lightweight tasks were sent to remote transcode servers, adding unnecessary overhead.
With fallback_rules, such detection jobs now run locally via fallback_ffmpeg, while normal transcodes continue to use remote hosts.

Key Points

• Uses substring matching on ffmpeg args (case-insensitive).
• Runs matched commands locally using fallback binaries.
• Fully backward-compatible when no rules are defined.

Example Log

[rffmpeg] Matched fallback rule: ['-vf blackframe', '-f null']