Add rate limiting for notifications

**Problem**
When working with core, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

The issue names no files, tests, notification entry point, or rate-limit policy. Start by locating notification handling in the repository and clarify the desired limit, scope, and verification criteria; the current description does not define what done looks like.

索引モデルが issue の本文から書いたものです。