Security fixes for OIDC authentication flow:
1. CSRF state validation (High): State nonce is now stored server-side
and validated on callback (single-use, 600s TTL)
2. PKCE S256 (Medium): code_challenge/code_verifier pair generated per
RFC 9126, sent in authorize URL and token exchange
3. Nonce in ID token (Medium): Random nonce included in authorize URL,
verified against ID token claims to prevent token replay
4. Secure token delivery (Medium): Tokens no longer in URL fragments.
One-time exchange code redirected to frontend, tokens retrieved via
POST /api/auth/oidc/exchange endpoint (60s TTL, single-use)
5. Registration guard (Low): POST /api/auth/register returns 403 when
disable_password_login is active in OIDC-only mode
Dionisio
1a1dee9179