- Replace ClaudeAdapter with GPTAdapter using OpenAI GPT (gpt-5-mini)
- Implement function calling for calendar operations (getDay, proposeCreate/Update/Delete, searchEvents)
- Add provider-agnostic AI utilities in ai/utils/ (systemPrompt, toolDefinitions, toolExecutor, eventFormatter)
- Add USE_TEST_RESPONSES env var to toggle between real AI and test responses
- Switch ChatService.processMessage to use real AI provider
- Add npm run format command for Prettier
- Update CLAUDE.md with new architecture
- Implement full CRUD in MongoEventRepository (findById, findByUserId, findByDateRange, update, delete)
- Extend ChatService to handle create/update/delete actions with dynamic test responses
- Add recurrenceExpander utility using rrule library for RRULE parsing
- Add eventFormatters utility for German-localized week/month overviews
- Add German translations for days and months in shared Constants
- Update client ChatService to support all event actions (action, eventId, updates params)
- Add AuthController login/register endpoints with error handling
- Implement AuthService with password validation and user creation
- Add MongoUserRepository with findByEmail and create methods
- Implement password hashing with bcrypt
- Add dotenv for environment variable support
- Add Docker Compose setup for MongoDB + Mongo Express
- Stub AuthMiddleware with fake user for testing
- Update CLAUDE.md with implementation status
