Add multi-provider LLM support with model selection

- Support OpenRouter, OpenAI, and Anthropic providers
- Fetch available models after entering API key
- OpenRouter sorted by price (cheapest first)
- Store provider, model, and key per user
- Update reflections API to use selected provider/model

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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