fix(backend): close model load races

Kokoro and LuxTTS load_model had no lock, so two concurrent requests
could both observe an unloaded model and double-load; they now use the
same double-checked asyncio.Lock pattern as the Chatterbox backends.
get_stt_backend gets the threading.Lock treatment the TTS and LLM
factories already had. Includes the ruff-era typing cleanup for
backends/__init__.

backend/backends/kokoro_backend.py

@@ -17,15 +17,12 @@ Languages supported (via misaki G2P):
 
 import asyncio
 import logging
-import os
-from typing import Optional
 
 import numpy as np
 
-from . import TTSBackend
 from .base import (
-    get_torch_device,
     combine_voice_prompts as _combine_voice_prompts,
+    get_torch_device,
     model_load_progress,
 )
 
@@ -122,8 +119,9 @@ class KokoroTTSBackend:
     def __init__(self):
         self._model = None
         self._pipelines: dict = {}  # lang_code -> KPipeline
-        self._device: Optional[str] = None
+        self._device: str | None = None
         self.model_size = "default"
+        self._model_load_lock = asyncio.Lock()
 
     def _get_device(self) -> str:
         """Select device. Kokoro supports CUDA and CPU. MPS needs fallback env var."""
@@ -157,7 +155,10 @@ class KokoroTTSBackend:
         """Load the Kokoro model."""
         if self._model is not None:
             return
-        await asyncio.to_thread(self._load_model_sync)
+        async with self._model_load_lock:
+            if self._model is not None:
+                return
+            await asyncio.to_thread(self._load_model_sync)
 
     def _load_model_sync(self):
         """Synchronous model loading."""
@@ -239,8 +240,8 @@ class KokoroTTSBackend:
         text: str,
         voice_prompt: dict,
         language: str = "en",
-        seed: Optional[int] = None,
-        instruct: Optional[str] = None,
+        seed: int | None = None,
+        instruct: str | None = None,
     ) -> tuple[np.ndarray, int]:
         """
         Generate audio from text using Kokoro.