address CodeRabbit review: fix 4 critical + 12 major issues

Critical:
- Remove dead backend.utils.validation PyInstaller hidden import
- Fix story_items table rebuild to preserve track/trim/version columns
- Guard cache migration against same source/destination path
- Fix regeneration audio overwrite (use random uuid suffix per take)

Major:
- Engine selector: validate language on Qwen switch, clear stale modelSize
- Sync language validation regex between profile create and generate (22 langs)
- Guard CUDA download against duplicate concurrent requests
- Only set model_size for engines that support multiple sizes
- Fix 404 swallowed by generic except in history export
- Validate audio_path before FileResponse in export-audio
- Transcription: stream uploads in 1MB chunks, use robust cache check,
  call complete_download() on Whisper download success
- Set clean version as default when effects chain validation fails
- Return explicit error when Windows port occupied by non-voicebox process

app/src/components/Generation/EngineModelSelector.tsx

@@ -42,8 +42,15 @@ function handleEngineChange(form: UseFormReturn<GenerationFormValues>, value: st
     const [, modelSize] = value.split(':');
     form.setValue('engine', 'qwen');
     form.setValue('modelSize', modelSize as '1.7B' | '0.6B');
+    // Validate language is supported by Qwen
+    const currentLang = form.getValues('language');
+    const available = getLanguageOptionsForEngine('qwen');
+    if (!available.some((l) => l.value === currentLang)) {
+      form.setValue('language', available[0]?.value ?? 'en');
+    }
   } else {
     form.setValue('engine', value as GenerationFormValues['engine']);
+    form.setValue('modelSize', undefined as unknown as '1.7B' | '0.6B');
     if (ENGLISH_ONLY_ENGINES.has(value)) {
       form.setValue('language', 'en');
     } else {