Add first backend integration tests for golden flow

[danieleforte92]

Goal

Add the first backend integration tests that validate the core golden flow end-to-end at API/service level.

Why

Current CI can run without tests. We need baseline coverage on the most critical behavior before expanding scope.

Scope

• Cover: stock setup -> recipe context -> cooking preview -> cooking commit
• Validate FEFO selection behavior
• Validate transaction/projection consistency after commit
• Use deterministic seed/test data

Acceptance criteria

• At least one happy-path integration test for the golden flow
• At least one edge-case test (insufficient stock or partial consumption)
• Tests run in CI backend job

Out of scope

• Full unit-test coverage of all services
• Frontend E2E tests