Add LLM Model Selector in UI Itself

[MODSetter]

Lets Add LLM Model Selector in UI Itself. We already have Connector management system, just need to implement LLM Model management system. I will do it myself. Open to any suggestions.

This is the rough plan for now:

1. Add LLMProvider TABLE & assiciate it with user
2. Add its crud routes.
3. At startup if no llmprovider for this user then redirect to create one. Also let them create more if needed.
4. Then Let them select our config vars ie fast, statergic, longconext llms.
5. Update all code to use that.
6. Add its management UI in User Settings > Settings
7. Test
8. Let new contributors add remaining providers (These will be good first issues), but I will make sure I have custom fallback to set any LLM in LiteLLM so Users can still add any LLM config.

[MODSetter]

https://github.com/MODSetter/SurfSense/pull/164

[Fau57]

Thanks for this

[MODSetter]

Thanks for this

Nice. It still need to be tested fully though. LMK if you face any problems configuring a LLM Provider through this.