Add server-side JWT authentication with permission-based access control
(read/write/delete claims). Password authentication now uses HTTP Basic
auth only (replacing Bearer). Add configurable username for both server
and client (--server-username/--client-username, defaults to "keep").
JWT secret supports file-based loading via --server-jwt-secret-file for
Docker secrets. OPTIONS preflight requests bypass auth. HEAD mapped to
read permission.
Co-Authored-By: opencode <noreply@opencode.ai>