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/components/shared/AudioSettingsModal.tsx

@@ -34,6 +34,10 @@ try {
 
 export type AudioGenerationSettings = {
   voiceId: string;
+  customVoiceId?: string;
+  useVoiceClone?: boolean;
+  voiceSampleUrl?: string;
+  voiceCloneEngine?: string;
   speed: number;
   volume: number;
   pitch: number;

frontend/src/components/shared/VoiceSelector.tsx

@@ -136,7 +136,7 @@ const PREDEFINED_VOICES: VoiceOption[] = [
   { id: "Exuberant_Girl", name: "Exuberant Girl", personality: "Joyful, expressive female voice - ideal for celebrations", previewUrl: VOICE_PREVIEW_MAP.Exuberant_Girl, gender: "female", category: "creative" },
 ];
 
-const VOICE_CLONE_ID = "MY_VOICE_CLONE";
+export const VOICE_CLONE_ID = "MY_VOICE_CLONE";
 
 export const VoiceSelector: React.FC<VoiceSelectorProps> = ({
   value,