Huzayfa El Hussein

A multi-tenant platform for German car dealerships covering inventory, customers, sales, invoicing and compliance. Built from scratch to replace a spreadsheet-and-paper workflow, which meant getting German vehicle tax, emissions and data-protection law right, not just building CRUD.

• Tenant isolation everywhere. Every repository query is scoped by dealership, with no unscoped finders exposed. This surfaced and closed a cross-tenant leak where one dealership could attach another's customer to a sale.
• One source of truth for permissions. Role rules live in a single policy map shared by the backend and a frontend hook, so authorization can't drift between the two layers.
• A German vehicle regulatory engine. Extracted the KBA type-approval register (37,864 rows) out of a published PDF, reverse-engineered the undocumented fuel-code table, and built a §9 KraftStG tax calculator verified to the euro against known cars. Values are derived at read time with a versioned rates stamp, so a change in tax law can never leave stale figures in the database.
• Document intake by photo. A custom-trained Google Document AI processor reads the Fahrzeugschein, and a backfill only fills empty fields, never overwriting what the dealer entered. This cut intake from roughly twenty typed fields to a photo and a quick review.
• Reliability at the edges. mobile.de publishing uses a transactional outbox so a listing is never lost or double-sent, and rate limiting runs as an atomic Redis Lua token bucket rather than a racy check in application code.

A shared household app for two people running a home together: weekly meal planning, a shared shopping list, and budgeting. Every piece of data belongs to a household rather than a user, so both partners edit one shared source of truth.

• One UI codebase across platforms. Android and iOS share a Compose Multiplatform UI. For a lists-and-forms app with no platform-specific interaction, maintaining two native UIs as a solo developer was the wrong cost.
• Refresh tokens that can actually be revoked. They're opaque random strings, not JWTs, stored as hashes in a rotation chain. Presenting an already-rotated token is treated as theft and revokes the whole chain.
• Membership as a lifecycle, not a foreign key. Leaving a household is a status change rather than a destructive delete, and the single-active-household rule is enforced by a Postgres partial unique index instead of every code path remembering to check.
• Right-to-left designed in, not bolted on. English, German and Arabic from the first commit, with a matched Arabic type family, adjusted line height, and rules for which icons mirror and which don't.
• Deliberate scope. No microservices, no bank integrations, no admin dashboard for a two-person product. Architectural effort went where reversal is expensive; everything else was cut to ship.