fix: restrict /force-election to localhost callers

[micahkendall]

Motivation

• The health server exposed an unauthenticated POST /force-election endpoint bound to 0.0.0.0 that could invoke assume_leadership with an attacker-chosen term and destabilize Raft availability.

Description

• Add ConnectInfo<SocketAddr> and import SocketAddr so the handler can inspect the caller's socket address.
• Use app.into_make_service_with_connect_info::<SocketAddr>() when serving the Axum router to populate connection info for handlers.
• Modify force_election to reject non-loopback callers with 403 FORBIDDEN and a warning log while preserving existing behavior for local callers.
• Keep all existing request/response semantics and call into raft_client.assume_leadership(req.term) for allowed requests.

Testing

• Ran cargo check against the workspace and it succeeded.

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Codex Task