feat(providers): add Featherless free tier provider

[gitcommit90]

Closes #2033

Summary

Adds Featherless as a first-class free-tier provider in the 9router provider registry.

Changes:

• — new provider module (base URL, headers, model-list normalisation)
• — register Featherless alongside existing providers
• — 4 unit tests covering endpoint construction, auth header injection, model-list normalisation, and error passthrough

Tests: 4/4 pass (npm test -- tests/unit/featherless-provider.test.js)