[PR #342] Implement OAuth 2.1 flow

Pull Request — Author: jpark (James Park)
Status: OPEN (14 days, NO reviewer assigned)

Implements the OAuth 2.1 authorization flow as specified in ADR-048.

This PR includes:
- PKCE support
- Removal of implicit grant
- Updated token endpoint

References: ADR-048 (Migrate to OAuth 2.1)
Blocked: No reviewer assigned for 14 days. This is blocking LIN-2001.

⚠️ NEEDS REVIEW — Critical for Q3 launch.