Implements GET /foundation/1.1/oauth2/auth_url, POST /oauth2/token, and
POST /oauth2/token_refresh, proxying to Forgejo's OAuth2 endpoints using
IFCURL_OAUTH2_CLIENT_ID and IFCURL_OAUTH2_CLIENT_SECRET env vars.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>