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/factory.py

@@ -77,6 +77,10 @@ def _stt(s: Settings):
 def _tts(s: Settings):
     if s.tts in (None, "none"):
         return None
+    if s.tts == "melo":
+        from .backends.melo import MeloTTS
+
+        return MeloTTS()
     from .backends.mock import MockTTS
 
     return MockTTS()