refactor: replace external memory backend with custom sqlite-vec storage engine featuring RRF search and Ebbinghaus decay

refactor: remove redundant self-import in core/memorize.py

refactor: remove redundant LLAMA_MODEL configuration from memorize and think modules

refactor: encapsulate memory management into AikoMemorize class and remove deprecated sqlite-vec implementation details from core/memorize.py

chore: update default USER_ID from OppaAI to Guest

Fix LLM call URL

Remove obsolete stuffs

Fix memory issue

Rename memorize.py to core/memorize.py