Configurable LLM model/provider + multi-library support (supersedes #2, #3, #4)

[LubergAlexander]

Summary

Consolidates and reimplements the good ideas from the three open PRs into one cohesive change, and adds the runtime-configurable model/provider/API key that ties them together. The originals overlapped heavily (#3 contained all of #2) and had quality issues, so this reimplements them cleanly with credit to the original authors.

Closes #2, #3, #4 (superseded).

What's included

🎵 Multiple music libraries — supersedes #2 (@othyn)

• Discovers every artist-type library section instead of assuming one named "Music".
• Caches artists across all libraries (de-duplicated by rating key) and searches across all of them.

🤖 Model selection + playlist metadata — supersedes #3 (@othyn)

• Playlist summary records provenance: Generated by Plexmuse + prompt + model.
• Bug fix: min_tracks/max_tracks were never interpolated into the track-recommendation system prompt (missing f-string) — now they are.
• UI model selector is driven by server config, not hardcoded gpt-4/gpt-3.5-turbo.

🔑 Robust API key handling — supersedes #4 (@quinneydavid)

• Validates the required provider key before every call. Critically, empty/whitespace *_API_KEY values are treated as missing — litellm.validate_environment alone reports an empty OPENAI_API_KEY= as present, which is the exact bug Fix handling of empty OpenAI API key #4 was chasing.
• Fails fast with a clear, actionable error naming the env var to set — no silent, hardcoded fallback to a (now-deprecated) Claude model.
• Centralized in a single _complete() helper, eliminating the duplicated completion() calls from the original PR.

⚙️ Configurable at Docker launch (new)

• PLEXMUSE_MODEL — default model; any LiteLLM string, so any provider (OpenAI, Anthropic, Gemini, …).
• PLEXMUSE_MODELS — optional comma-separated list offered in the UI dropdown.
• New GET /config endpoint + injected window config expose these to the frontend.
• Documented in README.md and .env.example.

docker run -p 8000:8000 \
  -e PLEX_BASE_URL=... -e PLEX_TOKEN=... \
  -e PLEXMUSE_MODEL=anthropic/claude-3-5-sonnet-latest \
  -e ANTHROPIC_API_KEY=sk-ant-... \
  ghcr.io/lubergalexander/plexmuse:latest

Notes / deviations from the originals

• Did not adopt the originals' CI/CD (ci-cd.yml) edits — they conflict with the current master latest-tagging approach.
• Did not adopt PR LLM selection (depends on #2) #3's wholesale Prettier reformat of app.js/index.html; kept changes minimal and in the existing style.

Testing

• Test suite: 21 → 32 passing, including new coverage for empty/missing API keys, multi-library init & de-dup, playlist metadata, config parsing, and /config.
• pylint 10.00/10; flake8, black --check, isort --check all clean.

🤖 Generated with Claude Code