Persist realtime transcription sessions to history

The websocket handler now accumulates the full session's PCM audio
(not just the per-chunk buffer that gets discarded after each
worker call) and, on stop, writes it as a wav to UPLOAD_DIR plus a
result JSON to RESULT_DIR via the new _save_realtime_history()
helper — same shape /transcribe already writes, tagged
gateway_backend="realtime" so the history table visually
distinguishes it from file uploads. This reuses /asr/history and
/asr/uploads/{filename} as-is; no new endpoints needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/ui/assets/js/asr.js

@@ -200,7 +200,7 @@ async function startRecording() {
       appendLive('');
     } else if (msg.type === 'final') {
       rtFinalText.value = msg.text || '';
-      setIndicator('idle', '완료');
+      setIndicator('idle', msg.id ? '완료 (처리 내역에 저장됨)' : '완료');
       cleanupAudio();
     } else if (msg.type === 'error') {
       setIndicator('idle', `오류: ${msg.detail}`);