provision ffmpeg on DeepDrftAPI host for Opus transcode

Phase 18.1 needs ffmpeg (libopus). Add it to bootstrap.sh apt prereqs and a
preflight guard in install.sh; resolves via the systemd user unit's default
PATH (/usr/bin), no config change.

deploy/bootstrap.sh

@@ -41,7 +41,8 @@ apt-get install -y --no-install-recommends \
   openssl \
   jq \
   wget \
-  ca-certificates
+  ca-certificates \
+  ffmpeg
 
 echo "  [ok] apt packages installed"