Add API backend and optimize whitelist fetching

- Added Node.js/Express API server with JWT authentication
- Implemented whitelist CRUD endpoints (GET, POST, PUT, DELETE)
- Added MongoDB integration with Mongoose
- Updated .gitignore for Node.js dependencies
- Optimized Android app whitelist fetching:
  * Two-level caching (HTTP + in-memory)
  * Exponential backoff retry logic
  * Automatic token refresh on 401 errors
  * Improved error handling and offline support
- Added API documentation and Postman collection
- Updated server IP to 192.168.2.244:5001

Author: Namitjain07

.gitignore

@@ -13,3 +13,12 @@
 .externalNativeBuild
 .cxx
 local.properties
+
+# Node.js (for api folder)
+node_modules/
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+package-lock.json
+.env
+.env.local