As a developer, I want to register my application as an OAuth client so that users can authorize my app to access their data using standard OAuth 2.0 flows.
## Acceptance Criteria
- [ ] Register OAuth 2.0 client application
- [ ] Support Authorization Code flow
- [ ] Support PKCE for public clients (mobile/SPA)
- [ ] Configure allowed scopes per client
- [ ] Consent screen customization
- [ ] Token endpoint for code exchange
- [ ] Refresh token support
- [ ] Client credentials flow for server-to-server