fix: add missing migrateConfig method, config versioning, and formatters module

ConfigManager.migrateConfig() was called but never defined — would crash at runtime.
Added config version checking, migration support, and fixed createEndpoint indentation.
New formatters module (csv, influxdb, json) for pluggable output formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/helper/formatters/jsonFormatter.js

@@ -0,0 +1,22 @@
+/**
+ * JSON formatter
+ * Produces a JSON string suitable for MQTT, REST APIs, etc.
+ *
+ * @param {string} measurement - The measurement name (e.g. node name)
+ * @param {object} metadata    - { fields, tags }
+ *   - fields: key/value pairs of changed data points
+ *   - tags:   flat key/value string pairs
+ * @returns {string} JSON-encoded string
+ */
+function format(measurement, metadata) {
+  const { fields, tags } = metadata;
+  const payload = {
+    measurement: measurement,
+    fields: fields,
+    tags: tags || {},
+    timestamp: new Date().toISOString(),
+  };
+  return JSON.stringify(payload);
+}
+
+module.exports = { format };