Remove duplicate class/function definitions from coderai

Removed ~1500 lines of duplicate code that now exist in codai modules:
- ModelCapabilities, detect_model_capabilities (now in codai.models.capabilities)
- Cache functions (now in codai.models.cache)
- detect_available_backends, check_flash_attn_availability (now in codai.backends)
- ModelBackend abstract class (now in codai.backends.base)
- ModelManager, WhisperServerManager, MultiModelManager (now in codai.models.manager)
- QueueManager (now in codai.queue.manager)
- Utility functions (now in codai.models.utils)

The code now properly imports from codai modules instead of having inline duplicates.