fix(tts): remove monster-voice artefact — disable librosa pitch shift, base speed 1.3→1.0

The emotional TTS path applied a librosa post-hoc pitch_shift (±1–3 semitones)
on top of a 1.3x-fast Melo base, producing a robotic "monster" delivery. Zero
out the pitch column for every emotion so pitch_shift is never invoked (the
worker's _pitch_shift already no-ops on 0.0), and drop the default synthesis
speed to 1.0. Emotion is now conveyed by speed alone — natural, artefact-free.
The pitch column is retained so a proper pitch method can be re-enabled later.

Verified: 29 tests pass; real 2-emotion synthesis on CUDA yields a clean wav
with pitch=0.0 on all segments.

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 1.3)
+    WSAI_TTS_SPEED     synthesis speed multiplier (default 1.0)
 """
 
 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", "1.3"))
+                           else os.environ.get("WSAI_TTS_SPEED", "1.0"))
         self.sink = sink or _log_sink
         self._proc: asyncio.subprocess.Process | None = None
         self._lock = asyncio.Lock()