feat: audio transcoding endpoint for offline downloads (MP3/AAC)

[mateof]

Summary

The TFM Audio PWA can now offer offline playlist downloads in MP3/AAC so lossless files (FLAC) take 5-8x less space on the phone. This PR adds the server side: an FFmpeg-based transcoding endpoint with disk caching, plus a capabilities endpoint so clients can detect whether FFmpeg is installed and warn the user.

Endpoints

• GET /api/mobile/stream/transcode/info — reports { ffmpegAvailable, formats }. Clients must check this (or handle the 501 below) and fall back to original-format downloads when FFmpeg is missing.
• GET /api/mobile/stream/tfm/{channelId}/{tfmId}/transcoded?format=mp3|aac&bitrate=64..320&fileName=... —
  1. Ensures the original is fully cached in _temp (sequential 512KB Telegram download, same cache file the streaming endpoints use).
  2. Transcodes with FFmpeg (libmp3lame / native aac), writing to a .part file and moving into place so partial results are never served.
  3. Serves the cached result via PhysicalFile with full Range support. Repeat requests hit the disk cache under _temp/transcoded.

Safeguards

• 501 Not Implemented when FFmpeg is not installed (checked once and cached; transcode/info reports it too).
• Per-target locks prevent duplicate transcodes of the same file/format/bitrate; a global semaphore caps concurrent FFmpeg processes at 2.
• FFmpeg process is killed (entire tree) and the partial output deleted if the client aborts; non-zero exit codes surface the stderr tail in the server log.
• MP3 output keeps tags and embedded cover art (-map 0:v? -c:v copy -disposition:v:0 attached_pic); AAC/m4a keeps tags (-map_metadata 0).

Notes

• First request for a track can take a while (Telegram download + transcode happen synchronously before the response); subsequent requests are instant.
• No new configuration: FFmpeg is resolved from PATH, matching the existing video transcoding feature.

Test plan

• GET /api/mobile/stream/transcode/info returns ffmpegAvailable: true on a host with FFmpeg, false without it.
• Request a FLAC track with ?format=mp3&bitrate=192: response is a valid MP3 (~5-8x smaller), with tags/cover; file appears under local/temp/_temp/transcoded/.
• Second request for the same track returns instantly (cache hit).
• ?format=aac produces a playable .m4a.
• Without FFmpeg installed, the endpoint returns 501 with a JSON error.

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