feat: lazy-load local TTS/STT models on first request

Local /synthesize and /transcribe endpoints now auto-load Kokoro/Whisper
models on first call instead of returning 503 when not pre-loaded at
startup. This allows switching between Local and OpenAI providers in the
Flutter test page without requiring server restart.

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

it0_app/lib/features/agent_call/presentation/pages/voice_test_page.dart

@@ -30,7 +30,7 @@ class _VoiceTestPageState extends ConsumerState<VoiceTestPage> {
   bool _playerReady = false;
   bool _recorderReady = false;
 
-  _VoiceProvider _provider = _VoiceProvider.local;
+  _VoiceProvider _provider = _VoiceProvider.openai;
 
   String _ttsStatus = '';
   String _sttStatus = '';

packages/services/voice-service/src/api/test_tts.py

@@ -5,12 +5,73 @@ import io
 import os
 import struct
 import tempfile
+import threading
 import numpy as np
 from fastapi import APIRouter, Request, Query, UploadFile, File
 from fastapi.responses import HTMLResponse, Response
 
 router = APIRouter()
 
+_loading_lock = threading.Lock()
+
+
+def _ensure_local_tts(app) -> bool:
+    """Lazy-load Kokoro TTS if not yet loaded. Returns True if available."""
+    tts = getattr(app.state, "tts", None)
+    if tts is not None and tts._pipeline is not None:
+        return True
+    with _loading_lock:
+        # Double-check after acquiring lock
+        tts = getattr(app.state, "tts", None)
+        if tts is not None and tts._pipeline is not None:
+            return True
+        try:
+            from ..tts.kokoro_service import KokoroTTSService, _patch_misaki_compat
+            from ..config.settings import settings
+            _patch_misaki_compat()
+            from kokoro import KPipeline
+            svc = KokoroTTSService(model=settings.kokoro_model, voice=settings.kokoro_voice)
+            svc._pipeline = KPipeline(lang_code='z')
+            app.state.tts = svc
+            print(f"[lazy] Local TTS loaded: {settings.kokoro_model} voice={settings.kokoro_voice}", flush=True)
+            return True
+        except Exception as e:
+            print(f"[lazy] Failed to load local TTS: {e}", flush=True)
+            return False
+
+
+def _ensure_local_stt(app) -> bool:
+    """Lazy-load faster-whisper STT if not yet loaded. Returns True if available."""
+    stt = getattr(app.state, "stt", None)
+    if stt is not None and stt._model is not None:
+        return True
+    with _loading_lock:
+        # Double-check after acquiring lock
+        stt = getattr(app.state, "stt", None)
+        if stt is not None and stt._model is not None:
+            return True
+        try:
+            from ..stt.whisper_service import WhisperSTTService
+            from faster_whisper import WhisperModel
+            from ..config.settings import settings
+            svc = WhisperSTTService(
+                model=settings.whisper_model,
+                device=settings.device,
+                language=settings.whisper_language,
+            )
+            compute_type = "float16" if settings.device == "cuda" else "int8"
+            try:
+                svc._model = WhisperModel(svc.model_name, device=svc.device, compute_type=compute_type)
+            except Exception:
+                if svc.device != "cpu":
+                    svc._model = WhisperModel(svc.model_name, device="cpu", compute_type="int8")
+            app.state.stt = svc
+            print(f"[lazy] Local STT loaded: {settings.whisper_model}", flush=True)
+            return True
+        except Exception as e:
+            print(f"[lazy] Failed to load local STT: {e}", flush=True)
+            return False
+
 _SAMPLE_RATE = 24000  # Kokoro native output rate
 
 
@@ -210,10 +271,11 @@ async function doRoundTrip(blob) {
 
 @router.get("/tts/synthesize")
 async def tts_synthesize(request: Request, text: str = Query(..., min_length=1, max_length=500)):
-    """Synthesize text to WAV audio."""
-    tts = getattr(request.app.state, "tts", None)
-    if tts is None or tts._pipeline is None:
-        return Response(content="TTS model not loaded", status_code=503)
+    """Synthesize text to WAV audio (lazy-loads local Kokoro model on first call)."""
+    loaded = await asyncio.get_event_loop().run_in_executor(None, _ensure_local_tts, request.app)
+    if not loaded:
+        return Response(content="Failed to load local TTS model", status_code=503)
+    tts = request.app.state.tts
 
     loop = asyncio.get_event_loop()
     def _synth():
@@ -242,10 +304,11 @@ async def tts_synthesize(request: Request, text: str = Query(..., min_length=1,
 
 @router.post("/stt/transcribe")
 async def stt_transcribe(request: Request, audio: UploadFile = File(...)):
-    """Transcribe uploaded audio to text via faster-whisper."""
-    stt = getattr(request.app.state, "stt", None)
-    if stt is None or stt._model is None:
-        return {"error": "STT model not loaded", "text": ""}
+    """Transcribe uploaded audio to text via faster-whisper (lazy-loads on first call)."""
+    loaded = await asyncio.get_event_loop().run_in_executor(None, _ensure_local_stt, request.app)
+    if not loaded:
+        return {"error": "Failed to load local STT model", "text": ""}
+    stt = request.app.state.stt
 
     # Save uploaded file to temp
     raw = await audio.read()