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.go

@@ -0,0 +1,51 @@
+// Command openapi writes the MusicServer API description without starting the server.
+//
+//	go run ./cmd/openapi -out .
+//
+// It writes openapi.json (OpenAPI 3.1) and openapi-3.0.json (OpenAPI 3.0, for tools
+// that don't support 3.1 yet). A running server serves the same files at /openapi.json
+// and /openapi-3.0.json.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"music-server/internal/server"
+)
+
+func main() {
+	out := flag.String("out", ".", "directory to write openapi.json and openapi-3.0.json to")
+	flag.Parse()
+
+	if err := run(*out); err != nil {
+		fmt.Fprintln(os.Stderr, "openapi:", err)
+		os.Exit(1)
+	}
+}
+
+func run(out string) error {
+	spec := server.OpenAPISpec()
+	v31, err := spec.MarshalJSON()
+	if err != nil {
+		return err
+	}
+	v30, err := spec.Downgrade()
+	if err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(out, 0o755); err != nil {
+		return err
+	}
+	for name, data := range map[string][]byte{"openapi.json": v31, "openapi-3.0.json": v30} {
+		path := filepath.Join(out, name)
+		if err := os.WriteFile(path, data, 0o644); err != nil {
+			return err
+		}
+		fmt.Println("Wrote", path)
+	}
+	return nil
+}