Add maintenance API endpoints

docs/CONFIGURATION.md

@@ -166,6 +166,19 @@ After factory reset:
 - Temperature names return to generic defaults
 - BMS is unconfigured until selected or manually entered
 
+## Backup And Restore
+
+Configuration backup includes controller config and saved STA WiFi networks.
+
+HTTP endpoints:
+
+    GET  /api/v1/config/export
+    POST /api/v1/config/import
+
+`config/export` includes saved WiFi passwords so the backup can be restored. Treat exported JSON as sensitive.
+
+`config/import` persists the restored config. If WiFi networks are imported, run `POST /api/v1/wifi/connect` afterward to attempt STA connection.
+
 ## Configuration Persistence
 
 Configuration is stored using ESP32 non-volatile storage.