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.

internal/server/openapi_test.go

@@ -46,6 +46,13 @@ func TestOpenAPISpecIsValid(t *testing.T) {
 	assert.Len(t, spec31.Paths, spec.Paths.Len())
 }
 
+func TestOpenAPISpecWithoutServerMatchesServedSpec(t *testing.T) {
+	served := newTestEnv(t).do(http.MethodGet, "/openapi.json").Body.Bytes()
+	offline, err := OpenAPISpec().MarshalJSON()
+	require.NoError(t, err)
+	assert.JSONEq(t, string(served), string(offline))
+}
+
 func TestOpenAPIDocumentsTheV1API(t *testing.T) {
 	spec := loadSpec(t, newTestEnv(t))
 

internal/server/routes.go

@@ -8,12 +8,25 @@ import (
 	"music-server/internal/server/middleware"
 
 	"github.com/a-h/templ"
+	"github.com/danielgtaylor/huma/v2"
 	"github.com/labstack/echo/v5"
 	echoMiddleware "github.com/labstack/echo/v5/middleware"
 	"go.uber.org/zap"
 )
 
 func (s *Server) RegisterRoutes() http.Handler {
+	e, _ := s.routes()
+	return e
+}
+
+// OpenAPISpec returns the API description. It needs no database or configuration:
+// the handlers are registered but never called.
+func OpenAPISpec() *huma.OpenAPI {
+	_, a := (&Server{}).routes()
+	return a.v1.OpenAPI()
+}
+
+func (s *Server) routes() (*echo.Echo, apis) {
 	e := echo.New()
 	e.HTTPErrorHandler = httpErrorHandler
 	if len(s.cfg.TrustedProxies) > 0 {
@@ -58,7 +71,7 @@ func (s *Server) RegisterRoutes() http.Handler {
 	for _, r := range e.Router().Routes() {
 		logging.GetLogger().Debug("Registered route", zap.String("method", r.Method), zap.String("path", r.Path))
 	}
-	return e
+	return e, a
 }
 
 func redirect(to string) echo.HandlerFunc {