Run the instance in private mode when instance_url is not configured: the API rejects anonymous requests except the auth-bootstrap set (sign-in, token refresh, instance profile/settings, SSO providers, share-link access) plus first-run user creation, and the web UI redirects anonymous visitors to /auth instead of /explore. Setting instance_url keeps the current public behavior. Access tokens and personal access tokens are never gated.
Enforcement lives in a shared Authorizer used by both the Connect interceptor and the gRPC-gateway middleware; the file server applies the same rule to public-memo attachments and avatars. Also merges the duplicated Authenticate/AuthenticateToUser token dispatch behind resolveBearer, dedups the AuthContext unauthenticated state, extracts the redirect decision into a pure shouldGatePrivateInstance helper, and prints the access mode at startup.
