API Problem Details Consistency + Input Validation Hardening

Goal: Standardize API error handling and validation across order routes.
Rules:
1. Audit orders.js and index.js.
2. If validation gaps exist, add robust validation middleware for route inputs.
3. Ensure all error responses follow RFC 7807 structure consistently.
4. If already compliant, add tests that enforce format and prevent regressions.
Definition of done:
1. Consistent error payload shape.
2. Validation coverage for key route inputs.
3. Tests pass.