✨(backend) switch transcription from Kyutai to Voxtral realtime

The Kyutai open-source model turned out not to be production-ready:
it caused disruptions in the production environment, especially on
long-running meeting sessions.

Switch to the Voxtral realtime model, which looks like a
credible competitor and behaves much better in our setup.

For now, the code handling the Voxtral realtime API lives directly
in the project. It could be extracted into an open-source package
later.

See PR #1277 for the full details of the implementation, proposed
by @cameldev.

src/frontend/src/features/subtitle/component/Subtitles.tsx

@@ -75,15 +75,15 @@ const useTranscriptionState = () => {
     const segment = segments[0]
 
     setTranscriptionSegments((prevSegments) => {
-      const existingSegmentIds = new Set(prevSegments.map((s) => s.id))
-      if (existingSegmentIds.has(segment.id)) return prevSegments
-      return [
-        ...prevSegments,
-        {
-          participant: participant,
-          ...segment,
-        },
-      ]
+      const existingIndex = prevSegments.findIndex(
+        (s: TranscriptionSegmentWithParticipant) => s.id === segment.id
+      )
+      if (existingIndex === -1) {
+        return [...prevSegments, { participant, ...segment }]
+      }
+      const next = prevSegments.slice()
+      next[existingIndex] = { ...next[existingIndex], ...segment }
+      return next
     })
   }