slskd: scaffold reverse-proxy sidecar

Add officer-slskd, a singleton sidecar that reverse-proxies to a
self-hosted slskd (Soulseek) instance and reports its loopback port to
the API on connect. All slskd knowledge (URL + API key) lives in the
sidecar; the platform is a thin auth+forward proxy for /api/slskd/* and
holds no slskd credentials. Mirrors the officer-vault pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/sidecar/protocol.ts

@@ -60,6 +60,8 @@ export type SidecarEvent =
   | { type: 'music:server'; port: number }
   // Vault — the sidecar reports where its Vaultwarden reverse-proxy HTTP/WS server is listening on connect
   | { type: 'vault:server'; port: number }
+  // slskd — the sidecar reports where its slskd reverse-proxy HTTP server is listening (random port) on connect
+  | { type: 'slskd:server'; port: number }
   // Generic
   | { type: 'error'; id?: string; error: string };