Fix Redis driver availability check and add automatic fallback

- Add detailed driver status checking (extension + server connectivity)
- Implement automatic fallback to 'database' when configured driver is unavailable
- Update admin UI to show detailed status and fallback warnings
- Prevent Redis/Memcached activation without proper PHP extensions
- Add driver status constants: READY, NO_EXTENSION, NO_SERVER, UNAVAILABLE

Fixes issue where Redis showed as 'available' but caused fatal errors
due to missing phpredis extension.

Author: Алексей Тихомиров