[Feature request] Allow users to select the AI completion model per chat

It would be very useful if Docmost could support multiple configured AI completion models and allow users to select which model should be used for a chat. At the moment, the completion model is configured globally through: Changing the model therefore requires changing the configuration and restarting/recreating the Docmost container. For self-hosted setups using Ollama, it is common to have several models installed at the same time. Different models can perform better for different tasks, especially in technical knowledge bases and RAG workflows. Proposed functionality An admin should be able to define which completion models are available in Docmost. For example: The user could then select the desired model directly in the AI Chat, similar to model selection in ChatGPT, Claude or other AI chat applications. Ideally, the configuration could support: • A default completion model for the workspace • An admin-defined allowlist of available models • A model…