fix(tts): lower base speed 1.5→1.2 — 1.5x slurred Korean pronunciation

User reported the 1.5x base made Korean pronunciation mushy/slurred. Dial the
default WSAI_TTS_SPEED back to 1.2: still noticeably faster than the original
1.0, but clear. Emotion multipliers scale off base as before.

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

wsai/backends/melo.py

@@ -12,7 +12,7 @@ Env:
     WSAI_MELO_DEVICE   cpu | cuda | auto  (default auto: GPU if torch sees one,
                        else CPU; the worker falls back to CPU if CUDA fails)
     WSAI_TTS_OUT_DIR   where wavs are written (default ~/.cache/wsai/tts)
-    WSAI_TTS_SPEED     synthesis speed multiplier (default 1.5)
+    WSAI_TTS_SPEED     synthesis speed multiplier (default 1.2)
 """
 
 from __future__ import annotations
@@ -93,7 +93,7 @@ class MeloTTS:
         self.out_dir = Path(out_dir or os.environ.get("WSAI_TTS_OUT_DIR")
                             or (Path.home() / ".cache/wsai/tts"))
         self.speed = float(speed if speed is not None
-                           else os.environ.get("WSAI_TTS_SPEED", "1.5"))
+                           else os.environ.get("WSAI_TTS_SPEED", "1.2"))
         self.sink = sink or _log_sink
         self._proc: asyncio.subprocess.Process | None = None
         self._lock = asyncio.Lock()