feat: add providerless params endpoint

[guillaumegay13]

Summary

• add providerless GET /api/v1/params/{model}.json responses shaped as { model, params }
• emit static dist/api/v1/params/*.json files during build and list the endpoint in the API index
• document API-key vs -subscription model slug contracts in README, llms output, and the usage guide
• guard against conflicting duplicate providerless model slugs so ambiguous responses cannot be generated silently

Validation

• npx vitest run tests/catalog.test.ts tests/server.test.ts tests/llms.test.ts
• npm test
• npm run typecheck
• npm run validate
• npm run guard:params
• npm run build
• npm run lint
• git diff --check

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