22. [M3] Implement API integration tests for end-to-end v1 flows

[gravity-api]

Goal

Validate critical API flows through integration tests against SQLite-backed runtime.

Scope

• Auth, orders, inventory, allocation, picking, packing, shipping, reporting.
• Authorization boundaries by role.

Implementation Tasks

• Add integration harness for test host + test SQLite DB.
• Implement full happy-path fulfillment scenario test.
• Implement negative tests for authz and invalid state transitions.
• Add regression coverage for CSV import and backup/restore APIs.

Acceptance Criteria

• Core /api/v1 flows pass in integration environment.
• Authorization restrictions are validated by tests.
• Key failure paths return expected status and error schema.