Senior Fullstack Developer — Technical Lead

About Buun

Buun is a fintech startup developing a payment infrastructure for the insurance industry. Our platform issues programmable Visa cards that allow policyholders to pay for insured services (such as health, travel assistance, mobility, or veterinary care) instantly, without advancing funds. By connecting insurance policies with real-time payment authorization, Buun digitizes the claims payment process, reduces fraud, and significantly improves customer experience.

You will be the most experienced technical profile in the dev team — the reference on both backend architecture and frontend quality. You will lead two developers (one backend specialist in Spain, one fullstack in Portugal) and work closely with the CTO on all technical decisions.

The Role

You own the full technical stack — backend engine and user-facing interfaces — and ensure the team delivers with consistency and quality:

Backend Leadership

• Architecture and implementation of the real-time authorization engine (p95 < 500ms)
• Data model design like card_mapping, auth_event, posting_ledger, idempotency_key
• Concurrency management and double-spend prevention (PostgreSQL / Redis locking)
• Payment processor webhook validation (HMAC), idempotency handling and replay protection
• Security: secrets management, full audit trail, PCI-DSS awareness
• Observability: OpenTelemetry, metrics, alerting, operational runbooks

Frontend & Product Ownership

• Architectural decisions on the React / Next.js frontend (insurer dashboard + cardholder portal)
• Review and validation of React Native mobile app code (iOS + Android) produced by the Portugal-based developer
• Establish and maintain shared component standards, TypeScript conventions, and code quality baselines
• Ensure consistent UX across web and mobile surfaces

Team Lead

• Technical mentoring for both junior developers: code reviews, best practices, skill development
• Coordinate work allocation across the distributed team (France, Spain, Portugal)
• Technical interface with the CTO on architecture decisions
• Onboarding and ramp-up of new team members

Tech Stack & Environment

Backend

• Java 17+ / Spring Boot 3.x — solid command required
• Advanced PostgreSQL: ACID transactions, FOR UPDATE locking, Flyway/Liquibase
• REST APIs + Webhooks (HMAC, idempotency, replay protection)
• Docker / Kubernetes — HA multi-instance deployment
• Redis (distributed locks, velocity counters)
• OpenTelemetry / Grafana / structured logging

Frontend

• React / Next.js — web dashboards and cardholder portal
• React Native — mobile app iOS + Android
• TypeScript — mandatory across all frontend code
• REST API integration from frontend (auth, authorization events, card management)

Environment

• Startup: fast decisions, strong ownership, immediate impact
• Documented architecture, complete technical spec — you build on solid foundations
• Distributed team: France (on-site lead) + Spain and Portugal (EOR, regular on-site visits)

Candidate Profile

Required

• 4+ years of backend Java/Spring Boot development experience, with production code
• Strong PostgreSQL skills: transactions, locking, query optimization
• Proven frontend experience: React and/or React Native in a production environment
• TypeScript fluency — you write typed code by default
• Experience on systems requiring reliability and error-case handling (retry, idempotency, circuit breaker)
• Security awareness: input validation, secrets management, audit logging
• Docker and cloud deployment experience

Strong Differentiator — What Really Matters for This Project

• Fintech or payments experience: payment webhooks, banking APIs, ledger, card processors (even brief, even at a services firm)
• Knowledge of authorization flows: ISO 8583, or any real-time authorization system
• PCI-DSS (basic awareness is sufficient)
• Experience leading a small distributed team

Soft Skills

• Ability to be a technical reference without being a manager — explain, document, review
• High autonomy and ability to make technical decisions under uncertainty
• Absolute rigor — a production bug in a Visa payment flow has real-world consequences
• Natural communicator across technical and non-technical stakeholders

What We Offer

• ESOPs: attractive stock option package — you are among the first technical employees at Seed stage, where value potential is highest. Favorable taxation after 3 years (12.8%).
• Hybrid work: up to 2 days remote per week — extended remote possible upon approval for specific situations (travel, personal commitments)
• Premium health insurance (Alan Optimal plan) — 50% covered by the company
• Meal vouchers
• Technically challenging project in a rare domain: real-time Visa authorization, financial ledger, PCI — a genuine CV differentiator