fix(config): implement cached config fallback on load failure

[hdkiller]

This PR introduces a caching mechanism to the configuration loader to improve stability.

Problem:
Currently, if the configuration file update fails or is momentarily inaccessible (e.g., during a write operation), the application throws a 500 error immediately after retries fail.

Solution:

• Implemented a cachedConfig variable in src/server/utils/config.ts.
• When a config is successfully loaded, it is cached.
• If loadConfig fails (even after retries), it now checks for a cached version.
• If a cached version exists, it is returned with a warning log, keeping the application running with the last known good state.
• If no cache exists (e.g. startup failure), the original error behavior is preserved.