feat: add ffmpeg audio trim / cut download

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.

Dockerfile

@@ -2,6 +2,7 @@ FROM python:3.12-slim
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     alsa-utils \
+    ffmpeg \
   && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app