Add OpenAI OAuth provider

Part of #2

Implement OpenAI-specific OAuth device flow.

**Endpoints:**
- Device code: `POST https://api.openai.com/v1/oauth/device/code`
- Token: `POST https://api.openai.com/v1/oauth/token`
- Refresh: `POST https://api.openai.com/v1/oauth/token` (grant_type=refresh_token)

**Scopes:**
- `api.read`
- `api.write`

**Reference:** https://platform.openai.com/docs/api-reference/authentication