Add JWT_SECRET_NEXT env var support for seamless JWT secret rotation:
- JwtStrategy: use secretOrKeyProvider to try primary then fallback key
- TokenService.verifyAccessToken(): dual-key fallback for internal callers
- Redis metric jwt_verify_with_next_total for monitoring cut-over progress
- Session revocation marker support restored in JwtStrategy.validate()
- Unit tests for all three verification scenarios (primary, fallback, both-fail)
- docs/security/secret-rotation.md runbook with step-by-step rotation procedure
Closes GOO-203.
Co-Authored-By: Paperclip <noreply@paperclip.ing>