Senior Full-Stack Developer: make a healthcare billing app trustworthy (Next.js + Supabase)

We're a US healthcare billing startup with a working application that looks finished but isn't fully. The screens, schema, and UI are solid. The write paths are not: multi-step operations run without transactions, failures get swallowed, and there are no tests. We just completed a 156-finding professional code review of our own product, so we know exactly what's wrong and exactly what to fix, in addition to any input you may provide. You will not be guessing. This is a 4-6 week, cleanup sprint to an MVP that can process real medical claims. You'd be the senior engineering hand on the project, working directly with the product owner (strong on systems thinking, UX, and QA; not a coder) and an AI-assisted review pipeline where every PR gets two independent automated reviews before human merge. WHAT YOU'LL BUILD - Rebuild ~15 core write paths as transactional Postgres functions (Supabase RPCs) with idempotency keys: visit confirmation, claim creation, charge editing, payment posting - A payment ledger where balances are derived from records, never stored and hand-updated - A claim state machine enforced in the database, with work queues and alerts computed from claim state on a schedule - Clearinghouse integration for claim submission, status acknowledgments, and 835 remittance posting. Primary target is a major clearinghouse's developer program (REST/web services with HMAC auth plus sFTP; docs under NDA). Fallback is an API-first clearinghouse with public docs - Audit logging via database triggers, RLS hardening, MFA enforcement, and a clean migration baseline with a real ledger of what ran where - CI from day one: typecheck, lint, migration bootstrap test, and one end-to-end test (a patient going from scanned sheet to claim) that gates every merge WHAT ALREADY EXISTS Next.js 16 + Supabase + Vercel. A well-designed schema (claims with line items, integer cents, org scoping on every table), about 80 built screens, working OCR intake for scanned rounding sheets, and a detailed spec corpus. Plus the full code review documenting every defect with file-and-line citations. You inherit a map, not a mystery. MUST-HAVES - Deep PostgreSQL: transactions, row-level security, triggers, migrations. Supabase experience strongly preferred - You've built something where two systems had to agree about money, and something proved they agreed (reconciliation jobs, ledgers, balancing checks) - Testing is how you work, not something you add when asked. You can point to a solo project with CI - TypeScript + Next.js fluency - 30+ hours/week available for the next 6 weeks. This does not work as one of five parallel gigs - Comfortable with HIPAA context: no real patient data in dev, BAA discipline, PHI never in logs NICE-TO-HAVES - Healthcare billing/RCM or clearinghouse experience (837, 835, 277 familiarity). Fintech reconciliation experience counts too - Experience integrating gated or NDA enterprise APIs - Working hours with some overlap to both US Pacific and SE Asia time WHAT THIS IS NOT Not a greenfield MVP race, and not a "ship features fast" gig. We're optimizing for the boring virtues: transactions, tests, receipts, and honest failure states. If that sounds tedious, this isn't your project. If that sounds like how software should be built, we'll get along. HOW WE'LL START An in depth discussion, and then possibly audition tasks. Strong audition leads directly into the sprint. TO APPLY Start your proposal with the word LEDGER so we know you read this, then answer the screening questions. Proposals that skip the questions get skipped. 1. Describe a system you built where two systems had to agree about money. How did you prove they agreed? 2. Our app marks a claim "submitted" by writing a status field. What would you build so that "submitted" means something? 3. A five-step operation succeeds through step 3 and fails at step 4. Walk through exactly what your code does next. 4. You're the only developer on a project. What do your testing and deployment look like before you write the first feature? 5. How many hours per week can you commit for the next 6 weeks, and what else are you working on during that time? Thank you.