Add a command to write the OpenAPI spec without a server

`just openapi [dir]` (go run ./cmd/openapi) writes openapi.json (3.1) and openapi-3.0.json without a database or configuration. A test checks it matches the spec the server serves.

cmd/openapi/main_test.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRunWritesBothVersions(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, run(dir))
+
+	for name, version := range map[string]string{"openapi.json": "3.1.0", "openapi-3.0.json": "3.0.3"} {
+		data, err := os.ReadFile(filepath.Join(dir, name))
+		require.NoError(t, err)
+		var spec struct {
+			OpenAPI string         `json:"openapi"`
+			Paths   map[string]any `json:"paths"`
+		}
+		require.NoError(t, json.Unmarshal(data, &spec), name)
+		assert.Equal(t, version, spec.OpenAPI, name)
+		assert.Contains(t, spec.Paths, "/api/v1/music/info", name)
+	}
+}