[Feature] Add pending training keys and fix neural network training issues

[Feature] Add pending training keys and fix neural network training issues

- Add pending_train_key() for version-independent training vector storage
- Fix variable shadowing bug where ann_trained callback was overwritten
- Add concurrent training prevention via learning_spawned check
- Replace assert with proper error handling for msgpack parsing
- Clean up pending keys after successful training
- Update controller endpoint to use pending keys for manual training
- Fix ev_base:sleep() to register with session events properly
- Update classifier_test.lua to support llm_embeddings classifier testing

Co-Authored-By: Claude <noreply@anthropic.com>