feat(server): public GET /health endpoint for uptime monitors

Returns 200 with {status: ok, version, db: ok} when SQLite is reachable,
503 when the DB probe fails. Unauthenticated so external monitors can
poll without credentials.

server/lib/server_web/router.ex

@@ -45,6 +45,12 @@ defmodule ServerWeb.Router do
     get "/hosts/:name", HostController, :show
   end
 
+  scope "/", ServerWeb do
+    pipe_through :api
+
+    get "/health", HealthController, :show
+  end
+
   if Application.compile_env(:server, :dev_routes) do
     import Phoenix.LiveDashboard.Router