refactor(server): migrate to pool-based crawler management

Replace crawler_manager.py with simpler crawler_pool.py implementation:
- Add global page semaphore for hard concurrency cap
- Implement browser pool with idle cleanup
- Add playground UI for testing and stress testing
- Update API handlers to use pooled crawlers
- Enhance logging levels and symbols

BREAKING CHANGE: Removes CrawlerManager class in favor of simpler pool-based approach

Dockerfile

@@ -162,6 +162,9 @@ RUN crawl4ai-doctor
 # Copy application code
 COPY deploy/docker/* ${APP_HOME}/
 
+# copy the playground + any future static assets
+COPY deploy/docker/static ${APP_HOME}/static
+
 # Change ownership of the application directory to the non-root user
 RUN chown -R appuser:appuser ${APP_HOME}