Raft::AdminOps

The mutating cluster-administration surface an admin endpoint needs from a node. T-free by construction — none of these depend on the state-machine command type, which lets HTTP::AdminHandler be a concrete (non-generic) class.

Included by Raft::Node(T). The threading contract still applies: these mutate node state without locks, so callers outside the node's owning fiber must arrange their own safety (e.g. run the HTTP server in the same execution context, or marshal calls onto the node's fiber).