Senior Full-Stack Engineer: Finish a Multi-Tenant Financial SaaS (Next.js, TypeScript)

Background Sentinel is a financial operations platform for Shopify brands. It connects to Xero (accounting) and Shopify (commerce), builds a live three-statement financial model, and uses AI to surface risks and recommend actions. The core is built and architecturally sound. I need one strong contractor to finish the last ~20%, hardening, testing, and polishing it into something paying customers can rely on. This is finishing and production hardening work, not greenfield design. You'll be working inside an existing codebase and getting it to a reliable MVP for the first users. Engagement at a glance - One senior contractor - 4-6 weeks, ~30 hrs/week - Start: immediately - Payment: hourly, fixed-price milestones, or a mix - Budget: Fixed price with milestones - Trial on a scoped issue before the full engagement - Continuation: strong likelihood of ongoing work as we onboard customers, as the product will need constant iteration What "done" looks like (deliverables) I've framed these as verifiable outcomes so we both know when a milestone is complete. - Backend, data & the model - API data flow between Xero, Shopify and our backend is correct and secure, verified by tests against real sync/webhook payloads. - The Postgres schema is structured to scale to thousands of tenants efficiently (indexing, query plans, partitioning where warranted). - The Supabase backend (Edge Functions, RLS, migrations) is clean, documented, and maintainable. - The financial model produces correct outputs, verified by an automated test suite with known-good fixtures. The model is detailed, multi-component, and currently runs in TypeScript inside Supabase Edge Functions. Part of the brief is to assess whether the compute-heavy runs should stay on Edge Functions or move to a dedicated runtime as tenant count grows, and to make a recommendation. Onboarding & integrations - A user can get from connection to a populated financial model in under 10 clicks — the setup flow needs simplifying from a UX standpoint. - Google Drive connection plus AI-based document parsing. - A bidirectional Google integration (with the same pattern extensible to social platforms later). - A clear, well-documented user audit trail. Front end Finish the UI: populate charts with real model output, ensure clean rendering across breakpoints, and tidy up the existing screens. What's already built (high level) - Enough is in place that this is genuinely a finishing job: - A live three-statement model (P&L, balance sheet, cash flow) running as a multi-phase Deno Edge Function pipeline. - A scenario engine and working-capital logic (inventory costing, reorder recommendations, cash timeline). - Xero + Shopify OAuth, sync and webhooks (including Shopify's mandatory GDPR webhooks). - Stripe subscription webhooks with plan-tier gating. - Multi-tenant Row-Level Security on every table. Detailed architecture docs will be shared with shortlisted candidates under NDA. Required skills - 7+ years full-stack. - TypeScript; Next.js App Router and React 19. - Multi-tenant SaaS hardening: rate limiting, audit logging, preventing IDOR and RLS bypass. - Playwright E2E testing. - Stripe webhooks and subscription lifecycle. Nice to have - Supabase: PostgreSQL, Row-Level Security, Edge Functions (Deno). - Xero or Shopify API integration experience. - Three-statement financial modelling or supply-chain finance. Note, bring the financial domain knowledge and will validate model outputs, so this is a bonus, not a gate. - Anthropic Claude API (prompt caching, structured outputs). - Recharts customisation. How to apply In your proposal, please include: - CV. - One paragraph on the most relevant SaaS or fintech product you've shipped, and your specific role on it. - A link to a PR, repo, or code sample where **you** implemented multi-tenant data isolation.