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))