Senior Apps Script/Node.js Developer: Secure API Wrapper & Licensing Logic

I am looking for a Senior Backend Developer to build a secure "Middleware" engine using Google Apps Script (Web App) or a Node.js environment. This engine will act as a bridge between a frontend requester and a 3rd-party financial REST API.

​Key Tasks:

​Listener Setup: Build a doPost(e) function to receive and parse incoming JSON data packets.

​Licensing Layer: Integrate a check against a small database (e.g., Firebase or a master sheet) to verify a "License Key" before processing any data.

​API Integration: Using provided documentation for a REST API, write the logic to POST transactions (Journal Entries, Checks, Deposits).

​Security: Ensure API keys provided in the request are handled securely and never logged.

​Response Logic: Return standardized Success/Error codes to the requester so the frontend can display them to the user.

​Requirements:

​Deep experience with REST API integrations (JSON, Auth Headers).

​Experience with Firebase or Supabase for license management.

​Focus on security and "clean" data processing.