Senior Full-Stack Developer (FastAPI + React/Next.js) — Pre-Launch Platform, 2-Month Intensive

B2B PropTech marketplace connecting property managers and estate agents with verified service providers. The platform is built but not yet in production — we need a sr full stack engineer to take over the existing codebase, fix complex integration and architecture issues, and get it to a genuinely launch-ready state. This is a demanding, high-intensity 2-month engagement. You'll be inheriting an existing, partially built codebase — not starting from a clean spec. We need someone who can read unfamiliar code quickly, diagnose why integrations aren't working correctly, and make sound architectural calls under time pressure, ahead of launch. Stack: Backend: Python, FastAPI Frontend: React.js, Next.js, TypeScript, Tailwind CSS Database: PostgreSQL Infra: AWS, Docker, Git Integrations: Stripe, REST APIs, third-party CRM/PMS What you'll actually be doing: Taking ownership of an existing pre-launch platform with known integration and architectural issues Diagnosing root causes — not just patching symptoms — across payments, third-party APIs, and data sync Fixing and hardening complex integrations (Stripe, CRM/PMS, third-party APIs) end-to-end before launch Reviewing existing architecture and flagging/fixing what won't hold up in production Working fast, but writing clean, reviewable code under pressure Who this is for: Engineers who've worked with messy, real-world codebases before — not just greenfield builds People comfortable with ambiguity: sometimes you'll get "this integration doesn't work correctly, here's the symptom" rather than a full spec Strong debuggers and system thinkers first, feature-builders second Requirements: 5+ years professional experience with Python/FastAPI and React/Next.js Proven experience debugging and fixing complex third-party integrations (payments, CRM/PMS, or similar) Strong REST API and PostgreSQL experience Comfortable working independently and making technical decisions with incomplete information Available for high-intensity, near full-time hours for the 2-month duration Some timezone overlap with UK working hours (BST) preferred Nice to have: Stripe Connect experience specifically Marketplace/SaaS platform experience Experience taking a pre-launch product from "mostly working" to production-ready Selection process: Initial technical assessment (architecture reading + debugging exercise on a representative problem) Paid trial task on a real integration issue Short technical interview with the founding team