[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/composables/useFeatureFlags.ts

@@ -14,7 +14,8 @@ export enum ServerFeatureFlag {
   ASSET_UPDATE_OPTIONS_ENABLED = 'asset_update_options_enabled',
   PRIVATE_MODELS_ENABLED = 'private_models_enabled',
   ONBOARDING_SURVEY_ENABLED = 'onboarding_survey_enabled',
-  HUGGINGFACE_MODEL_IMPORT_ENABLED = 'huggingface_model_import_enabled'
+  HUGGINGFACE_MODEL_IMPORT_ENABLED = 'huggingface_model_import_enabled',
+  ASYNC_MODEL_UPLOAD_ENABLED = 'async_model_upload_enabled'
 }
 
 /**
@@ -65,7 +66,6 @@ export function useFeatureFlags() {
       )
     },
     get huggingfaceModelImportEnabled() {
-      // Check remote config first (from /api/features), fall back to websocket feature flags
       return (
         remoteConfig.value.huggingface_model_import_enabled ??
         api.getServerFeature(
@@ -73,6 +73,15 @@ export function useFeatureFlags() {
           false
         )
       )
+    },
+    get asyncModelUploadEnabled() {
+      return (
+        remoteConfig.value.async_model_upload_enabled ??
+        api.getServerFeature(
+          ServerFeatureFlag.ASYNC_MODEL_UPLOAD_ENABLED,
+          false
+        )
+      )
     }
   })