feat(dashboard): live TTS voice controls (speed/word-gap/sentence-gap/pitch)

Adds a "봇 목소리(TTS) 조절" panel to the voice-server dashboard so the four
controls can be tuned from the browser instead of only via env vars:

- 4 sliders (glyph speed, word gap, sentence gap, pitch) with live labels
- 미리듣기: synthesises a sample with the slider values WITHOUT changing the
  live bot voice (new per-call overrides on MeloTTS.synth)
- 봇에 적용: commits the slider values to the live TTS instance; next reply uses
  them. 기본값 button resets to the manual defaults.

Backend: GET/POST /api/tts/settings (clamped to the manual ranges) and POST
/api/tts/preview (returns audio/wav). Panel shows only when tts is a real
(non-mock) backend.

Verified end-to-end against a live dashboard instance: page renders the panel,
GET returns defaults, POST applies+clamps (pitch 99->12), preview returns a
valid wav and leaves the live settings unchanged; 29 tests pass.

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

wsai/backends/melo.py

@@ -198,14 +198,30 @@ class MeloTTS:
             self._ready = True
             log.info("melo worker ready in %s ms on %s", self.load_ms, info.get("device"))
 
-    async def synth(self, text: str) -> str:
+    async def synth(
+        self,
+        text: str,
+        *,
+        speed: float | None = None,
+        word_gap: float | None = None,
+        sentence_gap: float | None = None,
+        pitch: float | None = None,
+    ) -> str:
         """Synthesize `text` to a wav and return its path (no sink). Reusable by
-        callers that want the wav directly (e.g. the Discord voice bridge)."""
+        callers that want the wav directly (e.g. the Discord voice bridge).
+
+        The four controls default to the instance settings but may be overridden
+        per call (used by the dashboard preview so tuning does not disturb the
+        live bot voice until explicitly applied)."""
         await self._ensure()
+        spd = self.speed if speed is None else float(speed)
+        wg = self.word_gap if word_gap is None else float(word_gap)
+        sg = self.sentence_gap if sentence_gap is None else float(sentence_gap)
+        pt = self.pitch if pitch is None else float(pitch)
         text = normalize_for_speech(text)
         # Split on [감정] tags: each tag steers pitch/speed for the text that
         # follows (and is itself not spoken); non-emotion brackets stay as words.
-        segments = parse_segments(text, self.speed)
+        segments = parse_segments(text, spd)
         self._n += 1
         out = str(self.out_dir / f"tts-{self._n:06d}.wav")
         if segments:
@@ -215,18 +231,18 @@ class MeloTTS:
                     for s in segments
                 ],
                 "out": out,
-                "word_gap": self.word_gap,
-                "sentence_gap": self.sentence_gap,
-                "pitch": self.pitch,
+                "word_gap": wg,
+                "sentence_gap": sg,
+                "pitch": pt,
             }
         else:  # empty/whitespace reply: keep legacy single-utterance behaviour
             payload = {
                 "text": text,
                 "out": out,
-                "speed": self.speed,
-                "word_gap": self.word_gap,
-                "sentence_gap": self.sentence_gap,
-                "pitch": self.pitch,
+                "speed": spd,
+                "word_gap": wg,
+                "sentence_gap": sg,
+                "pitch": pt,
             }
         req = json.dumps(payload)
         s = time.monotonic()