feat: Carry audio level and VAD flag in mediajson media events

[bgrozev]

Adds two optional fields to the mediajson Media:

• audioLevel — RFC 6464 audio level (0-127, i.e. -dBov: 0 loudest, 127 silence), as reported by the sender in the ssrc-audio-level RTP header extension.
• vad — RFC 6464 Voice Activity Detection flag.

Both are optional and omitted from the wire when absent (sender didn't include the extension), so this is backward compatible. Encoded as natural JSON number/boolean, not string-encoded like the VoxImplant-derived chunk/timestamp.

This lets JVB forward per-frame speech information to the transcription/translation peer (opus-transcriber-proxy), which can use it to gate audio and avoid sending silence to paid STT backends.

Consumer side: jitsi/jitsi-videobridge#2430 (same branch name, built together by CI).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.65%. Comparing base (1e2216b) to head (cec3eb9).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #237      +/-   ##
============================================
+ Coverage     34.56%   34.65%   +0.09%     
  Complexity      162      162              
============================================
  Files            36       36              
  Lines          1380     1382       +2     
  Branches        140      140              
============================================
+ Hits            477      479       +2     
  Misses          870      870              
  Partials         33       33              

Files with missing lines	Coverage Δ
...n/src/main/kotlin/org/jitsi/mediajson/MediaJson.kt	98.50% <100.00%> (+0.04%)	⬆️

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