Adds a Cut panel inside every player row (✂ Cut button in the actions
column opens the row and focuses the Start field). Users type start and
end times in m:ss or h:mm:ss format; Download cut triggers
GET /api/cut which runs ffmpeg -c copy (no re-encode) and streams the
result as an attachment.
Clicking an analysis chip now also pre-fills the cut panel start/end
with the loud-section boundaries.
Server switched to ThreadingHTTPServer so ffmpeg runs don't block
other requests. ffmpeg added to Dockerfile apt install.
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