feat(tts): add [기본] neutral emotion + raise base speed to 1.5x

User reported the emotion samples sounded the same and the speed change wasn't
noticeable. Two changes:
- Add a "base" (기본/neutral) emotion at speed 1.0x so the plain base voice can
  be selected explicitly via a [기본] tag and auditioned against the others.
- Bump the WSAI_TTS_SPEED default 1.15 -> 1.5 for a clearly faster base voice.
  Emotion multipliers scale off base, so every emotion speeds up together.

Also extends gen_emotion_samples.py to emit one wav per emotion (incl. 기본)
plus a stitched all-in-one, so each emotion can be delivered as a separate clip.

Verified: 29 tests pass; match_emotion('기본') == 'base'; per-emotion synthesis
at base 1.5 produces distinct clip durations (base 4.9s vs happy 3.3s).

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 1.15)
+    WSAI_TTS_SPEED     synthesis speed multiplier (default 1.5)
 """
 
 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", "1.15"))
+                           else os.environ.get("WSAI_TTS_SPEED", "1.5"))
         self.sink = sink or _log_sink
         self._proc: asyncio.subprocess.Process | None = None
         self._lock = asyncio.Lock()