feat: voice clone audio generation + podcast workspace architecture

- Voice clone integration: When user selects voice clone in Write phase,
  backend uses their uploaded voice sample + scene script text to generate
  audio via qwen3/minimax/cosyvoice voice clone APIs
- Multi-tenant workspace storage: All podcast assets (audio, video, images,
  charts) now use workspace-specific directories per user
- Chart preview improvements: Card-based B-Roll charts UI with thumbnails,
  takeaway text, and action buttons; public endpoint for image serving
- Voice clone caching: In-memory LRU cache for voice samples (avoids
  re-downloading per scene); frontend caches voice clone metadata
- Thread pool for voice clone: Audio generation uses ThreadPoolExecutor to
  avoid blocking the FastAPI event loop
- Auto-detect voice clone IDs (vc_*, MY_VOICE_CLONE) to route correctly
- DB fallback for voice sample URL: Fetches from ContentAsset if not passed
- Fixed API URL resolution for chart previews
- Fixed GlassyCard DOM warnings for motion props
- Fixed ScriptGenerationProgressView syntax error
- Fixed usePodcastWorkflow scriptData reference

frontend/src/api/brandAssets.ts

@@ -17,6 +17,7 @@ export interface VoiceCloneResponse {
   voice_name?: string;
   preview_audio_url?: string;
   asset_id?: number;
+  engine?: string;
   message?: string;
   error?: string;
 }

frontend/src/api/client.ts

@@ -461,9 +461,11 @@ aiApiClient.interceptors.response.use(
     }
 
     if (error.response.status >= 500) {
-      openBackendCooldown(`http_${error.response.status}`);
+      // Do NOT trigger cooldown for application-level 500 errors (e.g. TTS failures).
+      // Cooldown should only block for network connectivity issues (handled above).
+      // Application 500s should be handled by individual callers.
       return Promise.reject(
-        new ConnectionError('Backend server is experiencing issues. Please try again later.')
+        new ConnectionError(`Server error ${error.response.status}: ${error.response.statusText || 'Internal Server Error'}`)
       );
     }