fix: vad query param, session metadata logging, enhanced health endpoint

- Accept use_vad as query param (request.args) with form data fallback
- Session metadata passthrough already implemented (logs session_id, teacher_id)
- Health endpoint already enhanced (model_loaded, gpu_memory_used_gb, active_connections)

hybrid_server.py

@@ -859,7 +859,7 @@ print(transcription.text)</code></pre>
                 language = request.form.get('language', None)
                 task = request.form.get('task', 'transcribe')  # 'transcribe' or 'translate'
                 model_size = request.form.get('model', 'base')
-                use_vad = request.form.get('use_vad', 'true').lower() == 'true'
+                use_vad = request.args.get('use_vad', request.form.get('use_vad', 'true')).lower() == 'true'
                 
                 # For now, we'll use the shared transcriber regardless of the requested model size
                 # In the future, we could create different transcriber instances for different models