Senior Python Backend Engineer: tax-software API platform (5-month contract, then retainer)

We are a US tax firm based in Israel building our own client platform for the 2027 filing season: a Python backend that runs engagements through intake, document extraction, preparation in Wolters Kluwer CCH Axcess (through its REST API), human review, e-signature, payment and e-file release. The specification, data model, state machine and API design are written. The core integration scripts exist and work in production today; your job is to turn them into a reliable service. YOU WILL - Own the backend: FastAPI, Postgres, a job queue, workers, infrastructure as code on AWS (or an agreed managed platform), CI/CD. - Build the CCH API module as a job runner with concurrency limits, retries, idempotency, prompt handling and clean logging, against a mock server we provide the spec for. - Implement a PII vault with field-level encryption and strict read paths; structured logs with redaction; an append-only audit log; role-based access. - Integrate Stripe, an e-signature vendor, Google Drive, transactional email and the WhatsApp Business API. - Work with a second engineer on the portal and with an AI assistant that writes specs and reviews pull requests; report weekly to the firm owner. - Carry on-call during the filing season (February-April and June 2027) under a retained maintenance agreement after the build. MUST HAVE - 5+ years of Python backend work in production. - Postgres: schema design, migrations, transactions. - REST/OAuth integrations with third-party APIs that fail in interesting ways (rate limits, locks, partial failures). - Queues and background jobs; Docker and one major cloud. - Secrets management and the habit of never logging sensitive data. - Written English good enough to document what you build. - At least four hours a day of overlap with Israel working hours (UTC+2). NICE TO HAVE: fintech, payments, accounting or other regulated-data experience; LLM API experience (Anthropic or OpenAI SDKs, tool use); SOC 2 or similar compliance work; Terraform or CDK. TERMS: 5-month contract, December 1, 2026 to April 30, 2027, 30-40 hours per week, US$30-45/hour (or a fixed US$3,500-6,000/month for full-time availability), then a retained maintenance agreement priced separately. NDA and IP assignment required. No access to production data or credentials at any point: development and staging run on a mock server with synthetic data. PROCESS: short written application answering the three questions below, then a 30-minute call, then a paid trial task (10-15 hours, US$500 paid regardless of outcome, on the mock server with synthetic data), then start. PLEASE ANSWER IN YOUR APPLICATION: 1. Describe one third-party API integration you built that had to survive locks, rate limits or inconsistent responses, and what you did about it. 2. How do you keep secrets and personal data out of logs in a system you run? 3. Your availability from December 1, 2026 to June 30, 2027 and your overlap with UTC+2. Applications without the three answers are not reviewed. Not available to contractors located in Russia (payment and compliance constraints).