feat(backend): backfill error code on legacy error responses

backend/cmd/server/main.go

@@ -317,6 +317,9 @@ func main() {
 		EnableStackTrace: true,
 	}))
 
+	// Backfill `code` on legacy JSON error responses during migration period.
+	app.Use(middleware.ErrorCodeEnricher())
+
 	allowedOrigins := getEnv("CORS_ALLOWED_ORIGINS", "http://localhost:5000")
 	allowCredentials := allowedOrigins != "*"
 	app.Use(cors.New(cors.Config{