refactor(broker): group Relaycast integration under relaycast module

[willwashburn]

Problem

Relaycast-specific concerns are spread across modules such as auth.rs, relaycast_ws.rs, message_bridge.rs, multi_workspace.rs, parts of main.rs, and parts of snippets.rs. The current layout makes it hard to separate broker runtime logic from Relaycast transport/API integration.

Scope

Group Relaycast-facing code under a dedicated module tree.

Suggested shape:

src/relaycast/mod.rs
src/relaycast/auth.rs
src/relaycast/ws.rs
src/relaycast/http.rs
src/relaycast/bridge.rs
src/relaycast/workspace.rs
src/relaycast/commands.rs

The exact split can be adjusted during implementation, but broker runtime code should call a clear Relaycast boundary instead of depending on scattered modules.

Acceptance criteria

• Relaycast WebSocket, HTTP registration/token logic, auth/session parsing, event mapping, broker command mapping, and multi-workspace session code live under relaycast::* or another clearly named integration namespace.
• Broker runtime modules import Relaycast integration through a small, explicit surface.
• Existing event mapping and WebSocket tests move with the relevant Relaycast modules.
• No external protocol behavior changes.
• Relevant Rust tests pass.

Notes

This is primarily a module ownership cleanup. It should avoid changing the Relaycast wire protocol or retry behavior unless a bug is discovered and handled separately.