[feat] Add async model upload with WebSocket progress tracking (#7746)

## Summary
- Adds asynchronous model upload support with HTTP 202 responses
- Implements WebSocket-based real-time download progress tracking via
`asset_download` events
- Creates `assetDownloadStore` for centralized download state management
and toast notifications
- Updates upload wizard UI to show "processing" state when downloads
continue in background

## Changes
- **Core**: New `assetDownloadStore` for managing async downloads with
WebSocket events
- **API**: Support for HTTP 202 async upload responses with task
tracking
- **UI**: Upload wizard now shows "processing" state and allows closing
dialog during download
- **Progress**: Periodic toast notifications (every 5s) during active
downloads with completion/error toasts
- **Schema**: Updated task statuses (`created`, `running`, `completed`,
`failed`) and WebSocket message types

## Review Focus
- WebSocket event handling and download state management in
`assetDownloadStore`
- Upload flow UX - users can now close the dialog and download continues
in background
- Toast notification frequency and timing
- Schema alignment with backend async upload API

Fixes #7748

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7746-feat-Add-async-model-upload-with-WebSocket-progress-tracking-2d36d73d3650811cb79ae06f470dcded)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: GitHub Action <action@github.com>

src/platform/assets/components/UploadModelUrlInput.vue

@@ -55,7 +55,7 @@
         <p v-if="error" class="text-xs text-error">
           {{ error }}
         </p>
-        <p v-else class="text-foreground">
+        <p v-else-if="!flags.asyncModelUploadEnabled" class="text-foreground">
           <i18n-t keypath="assetBrowser.maxFileSize" tag="span">
             <template #size>
               <span class="font-bold italic">{{
@@ -77,6 +77,10 @@
 import InputText from 'primevue/inputtext'
 import { computed } from 'vue'
 
+import { useFeatureFlags } from '@/composables/useFeatureFlags'
+
+const { flags } = useFeatureFlags()
+
 const props = defineProps<{
   modelValue: string
   error?: string