feat(tts): real Korean TTS via persistent MeloTTS worker

Adds a MeloTTS backend that runs the model in its own melo311 interpreter
as a long-lived worker (melo_worker.py), loaded once and fed synthesis
requests over a stdin/stdout JSON protocol. fd1 is split from fd2 in the
worker so MeloTTS's stdout progress chatter can't corrupt the protocol.
Each speak() writes a wav and hands the path to a pluggable sink (the
Discord voice step will swap in "play into the call"). factory wires
tts=melo; pipeline.aclose now also tears down the tts worker.

Verified (CPU): model load ~7.9s once, then a short reply synthesizes in
~0.86s (within the ~1s budget); wav is valid 44.1kHz PCM. GPU (cuda) is
selectable via WSAI_MELO_DEVICE for lower latency, pending GPU approval.
7 smoke tests still pass.

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

wsai/pipeline.py

@@ -162,8 +162,10 @@ class Pipeline:
             await self.aclose()
 
     async def aclose(self) -> None:
-        for closer in (self.source, self.stt, self.text_channel):
-            if closer is not None:
+        # tts is included because a real TTS (e.g. MeloTTS) owns a worker
+        # subprocess that must be torn down; mock backends have no aclose.
+        for closer in (self.source, self.stt, self.text_channel, self.tts):
+            if closer is not None and hasattr(closer, "aclose"):
                 try:
                     await closer.aclose()
                 except Exception: