- PKCE (S256) in OAuth flow: code_verifier in oauth_states, code_challenge in auth URL
- CryptoService: AES-256-GCM encryption for OAuth provider tokens at rest
- OAuth redirect URL validated against OAUTH_ALLOWED_REDIRECT_DOMAINS
- CHAT_JWT_SECRET must differ from JWT_SECRET in production
- Migration script: cmd/tools/encrypt_oauth_tokens for existing tokens
- Fixes: VEZA-SEC-003, VEZA-SEC-004, VEZA-SEC-009, VEZA-SEC-010