Initial commit: split VNOX monorepo into VNOX-Server

Unified server binary (vnox-serverd) combining:
- Gateway (TCP, auth, channels, sessions, SQLite)
- Voice-node (UDP relay, Opus, jitter buffer)

Standalone gateway (vnox-gateway) and voice-node (vnox-voice-node) preserved as sub-crates.

gateway/src/admin/mod.rs

@@ -0,0 +1,77 @@
+//! HTTP admin server — exposes `/health` and `/metrics` endpoints.
+//!
+//! Runs on a separate TCP port (default 7601) so it does not interfere
+//! with the LNEx control plane on 7600. All endpoints are unauthenticated
+//! and intended for orchestrators (k8s liveness probes, Prometheus scrapers).
+
+use std::sync::Arc;
+use std::time::Instant;
+
+use axum::{Router, extract::State, http::StatusCode, response::IntoResponse, routing::get};
+use tokio::net::TcpListener;
+use tracing::{error, info};
+
+use crate::admin::metrics::Metrics;
+
+pub mod metrics;
+
+#[derive(Clone)]
+pub struct AdminState {
+    pub started_at: Instant,
+    pub node_name: String,
+    pub node_address: String,
+    pub private_mode: bool,
+    pub metrics: Arc<Metrics>,
+    pub sessions_count: std::sync::Arc<std::sync::atomic::AtomicUsize>,
+    pub channels_count: std::sync::Arc<std::sync::atomic::AtomicUsize>,
+}
+
+pub async fn run(bind: String, state: AdminState) {
+    let app = Router::new()
+        .route("/health", get(health))
+        .route("/metrics", get(metrics))
+        .route("/version", get(version))
+        .with_state(state);
+
+    let listener = match TcpListener::bind(&bind).await {
+        Ok(l) => l,
+        Err(e) => {
+            error!("admin server failed to bind {bind}: {e}");
+            return;
+        }
+    };
+    info!("admin HTTP listening on {bind}");
+    if let Err(e) = axum::serve(listener, app).await {
+        error!("admin server: {e}");
+    }
+}
+
+async fn health(State(s): State<AdminState>) -> impl IntoResponse {
+    let uptime = s.started_at.elapsed().as_secs();
+    let body = serde_json::json!({
+        "status": "ok",
+        "uptime_seconds": uptime,
+        "node": s.node_name,
+        "address": s.node_address,
+        "private_mode": s.private_mode,
+    });
+    (StatusCode::OK, axum::Json(body))
+}
+
+async fn version(State(_s): State<AdminState>) -> impl IntoResponse {
+    let body = serde_json::json!({
+        "name": "vnox-gateway",
+        "version": env!("CARGO_PKG_VERSION"),
+        "lnex_version": "1",
+    });
+    (StatusCode::OK, axum::Json(body))
+}
+
+async fn metrics(State(s): State<AdminState>) -> impl IntoResponse {
+    let body = s.metrics.render_prometheus(&s);
+    (
+        StatusCode::OK,
+        [("content-type", "text/plain; version=0.0.4")],
+        body,
+    )
+}