Enhance Cache Middleware for Improved Background Processing and Logging

- Introduced placeholder content for processing and error states to provide immediate feedback during background data generation.
- Implemented a locking mechanism to prevent duplicate background tasks for cache revalidation, ensuring efficient resource usage.
- Enhanced logging throughout the middleware to improve traceability, including detailed logs for cache hits, misses, and background fetches.
- Refactored cache handling logic to streamline the process of serving cached responses and managing stale data.
- Removed redundant cleanup calls during cache hits to optimize performance.

These changes aim to improve the overall efficiency and reliability of the cache middleware, particularly for the largest_perp_positions endpoint.

Author: goldhaxx