Extract shared `verifyWithRotation` helper and `makeSecretOrKeyProvider` into `jwt-rotation.ts` so both REST (passport-jwt strategy) and WebSocket (TokenService.verifyAccessToken) paths honour JWT_SECRET_PREVIOUS during secret rotation. Add env-validation for optional previous secrets and document the rotation policy for WebSocket sessions. Resolves GOO-237 Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>