feat: add OpenAI support to the recommendations feature

[ianchesal]

Summary

The recommendations feature currently uses Claude (Anthropic) exclusively. Adding OpenAI as an alternative provider would let users bring their own OpenAI API key instead of — or in addition to — an Anthropic one.

Motivation

Not everyone has an Anthropic API key. OpenAI's models (GPT-4o, o1, o3) are widely used and a natural alternative for this kind of taste-matching recommendation task.

Proposed approach

• Introduce a RECOMMENDATION_PROVIDER env var (anthropic | openai, defaulting to anthropic)
• Add OPENAI_API_KEY env var alongside the existing ANTHROPIC_API_KEY
• Abstract the recommendation call behind a provider interface in src/lib/recommendations.ts so the prompt logic is shared and only the API call differs
• Update .env.example and the README configuration table
• Graceful degradation: if neither key is set, the recommendations page returns 503 as it does today

Open questions

• Should both providers be supported simultaneously (let the user pick in the UI), or is env-var selection enough?
• Which OpenAI model is the right default? GPT-4o is the obvious choice but o1/o3 may give better reasoning for taste-matching.

[ianchesal]

Unlikely I'd do this, but if someone wants an easy one, this is it.