Flutter Multi-App System + Node.js Backend (API + Order Management

I already have three Flutter apps in development—a Customer app, a Scrapyard app and a Driver app—all sharing one business flow. What I am missing is a robust, well-structured backbone that lets those clients talk to each other in real time and walk every order through its life-cycle without breaking.

Here is the flow the system must respect: a customer creates an order, scrapyards bid, the customer accepts an offer and pays, the scrapyard uploads an invoice, I (as admin) approve the transfer, the order moves into execution, delivery is tracked, warranty takes effect and finally everything is archived. Your job is to provide (or fix) the backend so every one of those steps is reflected instantly across all three apps.

Tech preferences are firm: Node.js for the server, PostgreSQL for the database and push notifications for the real-time layer. If you need sockets or a message queue behind the scenes that is up to you, but the user-facing mechanism must be push.

What I expect from you • A clean, documented REST (or GraphQL if you can justify it) API that each Flutter client can consume. • An efficient PostgreSQL schema that mirrors the order lifecycle and is migration-friendly. • Proper state management: each status change must trigger the correct push notification and be visible to the other apps with minimal lag. • Clear error handling so no order gets stuck in limbo. • Help wiring the three existing Flutter projects to the new endpoints and testing the complete flow end-to-end. • Deployment scripts or instructions that let me reproduce the environment on staging and production.

Acceptance criteria

1. Creating, bidding, accepting, paying and archiving an order from the Customer app updates instantly in the Scrapyard and Driver apps.
2. Admin actions performed from the back-office dashboard are reflected in the mobile clients without manual refresh.
3. All API routes are covered by unit or integration tests and ship with Postman (or similar) collections.

When you respond, please skip generic copy-paste. Tell me: • The high-level architecture you would choose (frameworks, libraries, hosting). • How you will model and transition order states safely. • Rough timeline with key milestones. • Your cost breakdown.

I will shortlist only those proposals that answer those four points clearly. Budget: USD 250–750 Skills: JavaScript, Node.js, PostgreSQL, Backend Development, Flutter, API Development, REST API