Implement centralized LLM config management

[zaira-bibi]

LLM provider credentials are currently owned by the chat plugin — hardcoded to one key per provider, with no user-facing label, no sharing across plugins, and no platform-level place to manage them. This issue introduces a first-class LLMConfig resource: a named, per-user bundle of provider + model + API key that any plugin can reference by ID.

Problem

• Credentials live inside app/core_plugins/chat/ — any new plugin needing LLM access must duplicate or couple to chat
• Users can only have one active key per provider; no way to label or distinguish keys
• There is no management UI — API keys are buried inside individual plugin config forms

Solution

• Extract shared LLM infrastructure (providers.py, encryption.py, cache.py) out of the chat plugin into a new app/llm/ module
• Introduce LLMConfig — a named, soft-deletable credential row owned by the user
• Expose full CRUD at /api/v1/llm/configs
• Add a dedicated LLM Configs settings page where users manage all their credentials in one place
• Update plugin config forms (chat, Slack) to select a config from a dropdown instead of entering raw API keys