Add launcher engine (scopenet-core) and admin panel server

- scopenet-shared: wire types for branding, auth, instances, manifests
- scopenet-core: Mojang versions/libraries/assets, automatic Java runtimes,
  Fabric/Quilt/Forge/NeoForge installation, file sync, launch command
  builder, Microsoft auth, servers.dat injection, server list ping
- scopenet-panel: Axum + SQLite admin API with JWT auth, users/groups,
  branding, settings, instances, Modrinth/CurseForge/zip import, file
  hosting and launch stats

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ARcGWxLx21FwXJ3yfGriS

panel/server/src/main.rs

@@ -0,0 +1,59 @@
+use scopenet_panel::{app, bootstrap_admin, build_state, config::Config, db};
+use tracing_subscriber::EnvFilter;
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    // `scopenet-panel healthcheck` — used by the Docker HEALTHCHECK so the
+    // image doesn't need curl.
+    if std::env::args().nth(1).as_deref() == Some("healthcheck") {
+        return healthcheck().await;
+    }
+
+    tracing_subscriber::fmt()
+        .with_env_filter(EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info,tower_http=warn,sqlx=warn")))
+        .compact()
+        .init();
+
+    let cfg = Config::from_env();
+    let pool = db::connect(&cfg.data_dir).await?;
+    if !cfg.web_dir.join("index.html").exists() {
+        tracing::warn!("web UI not found at {} (build panel/web or set SCOPENET_WEB_DIR)", cfg.web_dir.display());
+    }
+    let bind = cfg.bind.clone();
+    let state = build_state(cfg, pool).await?;
+    bootstrap_admin(&state).await?;
+
+    let listener = tokio::net::TcpListener::bind(&bind).await?;
+    tracing::info!("ScopeNet panel v{} listening on http://{bind}", env!("CARGO_PKG_VERSION"));
+    axum::serve(listener, app(state)).with_graceful_shutdown(shutdown()).await?;
+    Ok(())
+}
+
+async fn shutdown() {
+    let ctrl_c = async { tokio::signal::ctrl_c().await.ok(); };
+    #[cfg(unix)]
+    let term = async {
+        if let Ok(mut s) = tokio::signal::unix::signal(tokio::signal::unix::SignalKind::terminate()) {
+            s.recv().await;
+        }
+    };
+    #[cfg(not(unix))]
+    let term = std::future::pending::<()>();
+    tokio::select! { _ = ctrl_c => {}, _ = term => {} }
+    tracing::info!("shutting down");
+}
+
+async fn healthcheck() -> anyhow::Result<()> {
+    use tokio::io::{AsyncReadExt, AsyncWriteExt};
+    let bind = std::env::var("SCOPENET_BIND").unwrap_or_else(|_| "0.0.0.0:8080".into());
+    let port = bind.rsplit(':').next().unwrap_or("8080");
+    let mut s = tokio::net::TcpStream::connect(format!("127.0.0.1:{port}")).await?;
+    s.write_all(b"GET /healthz HTTP/1.0\r\nHost: localhost\r\n\r\n").await?;
+    let mut buf = String::new();
+    s.read_to_string(&mut buf).await?;
+    if buf.starts_with("HTTP/1.1 200") || buf.starts_with("HTTP/1.0 200") {
+        Ok(())
+    } else {
+        anyhow::bail!("unhealthy: {}", buf.lines().next().unwrap_or(""))
+    }
+}