[FEATURE]: Inject LLM API keys at user discretion via UI upon startup

[veekaybee]

Motivation

We currently inject OpenAI and MIstral API keys via .env/local environment variables into the application. See #599 and #609

We'd like for the user to actively inject these directly via UI.

This would involve:

1. Adding a route to add keys in the front-end
2. Accepting user settings to be passed in via API instead of via environment variables

Alternatives

No response

Contribution

No response

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