feat(stt-log): log the WHOLE turn pipeline to the transcript channel

The transcript channel only showed successful transcripts, so dropped utterances
(the 47/50 misses) were invisible. Now every captured utterance is mirrored with
its outcome and per-stage timing:
- too-short blip (<300ms), VAD "음성 아님(VAD 차단)", "인식 실패", "답변 없음", or "ok"
- transcript + reply (or "(무응답)")
- ⏱️ stt/llm seconds

The bridge meta now carries note + stt_sec + think_sec; voice.ts fires onTurn for
every turn (not only non-empty transcripts) and for the too-short drop; userbot
formats the diagnostic line.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

bot/src/bridge.ts

@@ -45,6 +45,11 @@ export interface ConverseMeta {
   reply: string;
   error?: string | null;
   broadcast_action?: BroadcastAction | null;
+  /** Why this turn produced (or didn't produce) a transcript/reply. */
+  note?: string;
+  /** Per-stage timing (seconds) for diagnosing latency. */
+  stt_sec?: number;
+  think_sec?: number;
 }
 
 export interface ConverseStreamHandlers {