v3.2.1 - Z.AI/GLM Model Routing Fix

- Add Z.AI provider routing for GLM models (glm-4.5, glm-4.6, etc.)
- Fix Settings UI integration for Z.AI API key storage
- API keys stored via Settings now work correctly with chat
- Add model parameter to chat endpoints for provider detection

- get_zai_token() now reads from user_api_keys table (Settings UI storage)
- Falls back to provider_connections table for legacy compatibility
- Frontend sends selected model to backend for correct routing