Epic: OAuth Provider Integrations (OpenAI, Copilot, etc.)

[Hozyne-OpenBak]

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

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Provider: GitHub Copilot (from #9)

• Device code + token exchange endpoints
• API base URL: https://api.githubcopilot.com/
• Reference: https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#device-flow

(Former issue #9 merged here.)