feat(stt): real Korean STT via persistent faster-whisper worker

Step 3 (귀): add WhisperSTT + whisper_worker, a warm out-of-venv worker
mirroring the MeloTTS shape (whisper312 venv, small/int8 on CPU). transcribe()
closes the voice round trip (MeloTTS wav -> whisper text); utterances() turns an
injected audio_source into Utterances (Discord voice feed pending). Wired into
factory as WSAI_STT=whisper.

Also address the arbiter's TTS follow-ups:
- melo worker error handling: capture stderr (drained in a bounded background
  task so the pipe can't fill), surface the real failure cause, and defend
  against an empty/invalid ready line instead of dying on JSONDecodeError.
- pipeline pre-warm: load slow backends (warmup()) at startup so the first
  utterance is answered warm; a warmup failure is logged, not fatal.

Verified: real TTS->STT round trip recovers the sentence near-perfectly;
warm transcribe ~1.2s (CPU). 12 tests pass.

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

wsai/pipeline.py

@@ -136,6 +136,28 @@ class Pipeline:
             await self._handle(utt)
 
     # -- lifecycle --------------------------------------------------------- #
+    async def _prewarm(self) -> None:
+        """Load slow-to-start backends before the loops accept input.
+
+        Real STT/TTS backends (faster-whisper, MeloTTS) load a model into a
+        persistent worker on first use — several seconds on CPU. Warming them
+        here means the first real utterance is answered warm (~1s) instead of
+        paying the cold model load mid-conversation."""
+        warmers = []
+        for comp, name in ((self.tts, "tts"), (self.stt, "stt")):
+            warmup = getattr(comp, "warmup", None)
+            if callable(warmup):
+                warmers.append((name, warmup))
+        if not warmers:
+            return
+        for name, warmup in warmers:
+            try:
+                await warmup()
+            except Exception as exc:  # a warm failure must not abort startup
+                log.warning("prewarm %s failed: %s", name, exc)
+                if self.monitor is not None:
+                    self.monitor.log("error", f"{name} 예열 실패: {exc}")
+
     async def run(self) -> None:
         # A TaskGroup (not bare gather) so that if ONE loop raises, the others
         # are cancelled and awaited before teardown. With plain gather the
@@ -145,6 +167,7 @@ class Pipeline:
         if self.monitor is not None:
             self.monitor.set_status(running=True)
             self.monitor.log("info", "파이프라인 시작")
+        await self._prewarm()
         try:
             async with asyncio.TaskGroup() as tg:
                 tg.create_task(self._perceive())