Need someone in HCMC, Vietnam - Troubleshooting for Educational website, setting up subscriptions with Stripe

Project Title: Full-Stack Developer for AI-Powered TOEFL Speaking App (Gemini AI + Stripe Integration)

Project Description:

I am looking for an experienced Full-Stack Developer to help me finalize and monetize an AI-driven English learning application (TOEFL Speaking Practice).

The app is currently a high-fidelity prototype built with Vanilla JS, Tailwind CSS, and Google Gemini AI (v1beta). It handles Text-to-Speech (TTS), audio transcription, and automated grading/feedback.

Key Tasks:

Stripe Integration: Implement a secure payment gateway (Stripe Checkout or Elements) to handle subscriptions or "pay-per-use" credits.

Backend Setup: Move the current client-side API logic to a secure backend (Node.js/Express, Firebase Functions, or Netlify Functions) to protect my Gemini and Stripe Secret Keys.

User Access Control: Ensure AI features are locked behind the payment gateway.

UI/UX Polishing: Improve the "Loading" and "Buffering" states for a smoother user experience on mobile and desktop.

Bug Squashing: Optimize the current PCM-to-WAV audio conversion logic to ensure zero-latency playback across all browsers (Safari/Chrome/Firefox).

Technical Stack:

Frontend: HTML5, Tailwind CSS, JavaScript (Vanilla).

AI: Google Gemini 2.5 (Flash & TTS).

Payments: Stripe API.

Backend: Open to suggestions (Node.js, Firebase, or Netlify preferred).

Requirements:

Proven experience with Stripe API (Subscribtions/Webhooks).

Experience working with AI APIs (Google Gemini, OpenAI, or Anthropic).

Strong understanding of Web Audio API and handling base64/binary audio data.

Ability to write clean, documented code.

Deliverables:

Fully functional payment flow.

Secure environment for API keys.

Deployed and tested version of the app.