Build Auth + Payments + Usage-Credit Layer for FastAPI/React SaaS (Stripe, Clerk, Postgres)

What this is: I'm the founder of a working AI SaaS platform (FastAPI/uvicorn backend, React/Vite frontend, deployed on AWS). The product pipeline is built and live! I need an experienced engineer to build the authentication, payments, and usage-entitlement layer that gates the product behind login and paid usage. You'll work against a runnable stub repository (real routes + data shapes, proprietary internals removed) - so you can build and test the full layer without ever touching the core IP, and afterwards - I will merge your work into the production app! Scope: • Auth (Clerk): signup/login, session lifecycle, backend session/JWT verification, inject a validated `user_id` into protected endpoints. • Payments (Stripe): Checkout for purchasing usage credits, plus a webhook that credits the user's ledger on successful payment. The webhook MUST be signature-verified, idempotent (Stripe retries-credit exactly once), and reconcilable (handle webhook-failed-but-payment-succeeded). • Entitlement ledger (the core): a credit balance per user with atomic check-and-decrement (two concurrent requests must never double-spend) on PostgreSQL. A `check_entitlement(user_id, ...)`gate that expensive endpoints call before doing paid work. • Server-side route gating: enforce entitlement on every paid endpoint (not just hidden in the UI); clean 401/402/403 responses. • Frontend integration (React): login/signup UI, buy-credits/checkout flow, balance display,graceful "out of credits" states, wired to the above. • Tests for the money-critical paths (webhook idempotency, atomic decrement, gating-bypass attempts). Stack you'll work with: Python 3.11, FastAPI, Pydantic, uvicorn (Linux). React + Vite, Tailwind. PostgreSQL. Clerk. Stripe. Who I'm looking for: Someone who has shipped this exact kind of system to production before. In your proposal, briefly tell me: 1. How you ensure Stripe webhook idempotency and handle reconciliation when a webhook fails but payment succeeded. 2. How you'd implement the atomic credit check-and-decrement so concurrent requests couldn't double-spend. 3. A relevant production system you've built (Stripe + auth + usage metering ideally). *I'd much rather a short, specific proposal - than a long generic one!* Working with me: I'm an engaged founder who genuinely enjoys building, and I want this to be a good experience for both of us! I'm responsive, communicate clearly, and I respect your expertise - because you're the specialist here,and I'm hiring you because you know this much better than I do! I'm not a micromanager and I'm not nitpicky - I just want the end results to be solid! I review in milestones, asking questions because I like understanding the work (not because I'm second-guessing you), and I make decisions quickly so you're never stuck waiting on me! The scope is already well-defined (detailed spec + a runnable stub repo on engagement), so you're not pricing around a vague, shifting target - it's a clean, bounded build. I care about getting the money-handling pieces right, and I'm betting you do too - if that resonates, I think we'll work well together! IP / engagement: A mutual NDA + IP-assignment (work-for-hire) before repo access. You build against stubs; you won't receive the proprietary pipeline. Work is milestone-based, fixed-bid preferred. To apply: Answer the three questions above + share 1-2 relevant examples, and ballpark your timeline and milestone structure (suggested: auth → payments+webhook → ledger+gating → frontend → tests/docs)!