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

backend/api/podcast/models.py

@@ -223,6 +223,9 @@ class PodcastAudioRequest(BaseModel):
     text: str
     voice_id: Optional[str] = "Wise_Woman"
     custom_voice_id: Optional[str] = None  # Voice clone ID for custom voice
+    use_voice_clone: Optional[bool] = False  # If True, use voice clone with voice_sample_url
+    voice_sample_url: Optional[str] = None  # URL to user's voice sample for cloning
+    voice_clone_engine: Optional[str] = None  # Engine: "qwen3", "minimax", "cosyvoice"
     speed: Optional[float] = 1.0
     volume: Optional[float] = 1.0
     pitch: Optional[float] = 0.0