feat(voice): improve TTS logging for root-cause diagnosis

- Route now logs every synthesis request (char count, voice, speed)
- Route logs char count + text preview when the 8000-char limit is hit
- Route logs empty audio with preview (helps spot no-chunk-produced edge case)
- Route logs success with byte count and duration
- Kokoro synthesise() logs per-call: samples produced, elapsed, chars/s
- Kokoro synthesise() logs warning when zero audio chunks returned with preview
- Kokoro synthesise() catches and logs pipeline-internal errors with preview
- Frontend: console.warn now includes char count + 80-char preview on failure and retry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/routes/voice.py

@@ -117,7 +117,12 @@ async def synthesise_speech():
     if not text:
         return jsonify({"error": "text is required"}), 400
 
-    if len(text) > 8000:
+    char_count = len(text)
+    if char_count > 8000:
+        logger.warning(
+            "TTS request rejected: text too long (%d chars, limit 8000). Preview: %r",
+            char_count, text[:120],
+        )
         return jsonify({"error": "text too long (max 8000 characters)"}), 400
 
     voice = str(data.get("voice", "af_heart"))
@@ -154,11 +159,29 @@ async def synthesise_speech():
         except Exception:
             pass
 
+    blend_desc = f"blend({len(voice_blend)} voices)" if voice_blend else voice
+    logger.info("TTS synthesis start: %d chars, voice=%s, speed=%.2f", char_count, blend_desc, speed)
+    t0 = time.monotonic()
     try:
         wav_bytes = await synthesise(text, voice=voice, speed=speed, voice_blend=voice_blend)
     except Exception:
-        logger.exception("TTS synthesis failed")
+        logger.exception(
+            "TTS synthesis failed: %d chars, voice=%s. Preview: %r",
+            char_count, blend_desc, text[:120],
+        )
         return jsonify({"error": "Synthesis failed"}), 500
 
+    duration_ms = round((time.monotonic() - t0) * 1000)
+    if not wav_bytes:
+        logger.warning(
+            "TTS synthesis returned empty audio: %d chars, voice=%s, %dms. Preview: %r",
+            char_count, blend_desc, duration_ms, text[:120],
+        )
+    else:
+        logger.info(
+            "TTS synthesis complete: %d chars → %d bytes in %dms (voice=%s)",
+            char_count, len(wav_bytes), duration_ms, blend_desc,
+        )
+
     from quart import Response
     return Response(wav_bytes, mimetype="audio/wav")