extend chat model with CRUD actions for event changes

- Add ProposedEventChange type with create/update/delete actions
- Replace proposedEvent with proposedChange in ChatMessage
- Add currentDate to AIContext for time-aware AI responses
- Add AI test endpoint for development (/api/ai/test)
- Fix MongoUserRepository type safety with explicit toUser mapping
- Update CLAUDE.md documentation

package-lock.json

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