feat(queue): implement queue scheduler service and API routes

- Added QueueSchedulerService to manage task queue lifecycle, including state machine, dependency resolution, and session management.
- Implemented HTTP API endpoints for queue scheduling:
  - POST /api/queue/execute: Submit items to the scheduler.
  - GET /api/queue/scheduler/state: Retrieve full scheduler state.
  - POST /api/queue/scheduler/start: Start scheduling loop with items.
  - POST /api/queue/scheduler/pause: Pause scheduling.
  - POST /api/queue/scheduler/stop: Graceful stop of the scheduler.
  - POST /api/queue/scheduler/config: Update scheduler configuration.
- Introduced types for queue items, scheduler state, and WebSocket messages to ensure type safety and compatibility with the backend.
- Added static model lists for LiteLLM as a fallback for available models.

ccw/frontend/src/components/shared/ConversationCard.tsx

@@ -198,7 +198,9 @@ export function ConversationCard({
 
             {/* Prompt preview */}
             <p className="text-sm text-foreground line-clamp-2 mb-2">
-              {execution.prompt_preview}
+              {typeof execution.prompt_preview === 'string'
+                ? execution.prompt_preview
+                : JSON.stringify(execution.prompt_preview)}
             </p>
 
             {/* Meta info */}