Add integration test for the invoice list endpoint filtering correctly by status

[Chucks1093]

Summary

The invoice list endpoint accepts an optional ?status query param. An integration test should seed invoices across all statuses and confirm the filter returns only invoices in the requested state without leaking invoices in other states.

Scope

• Seed one invoice each in: draft, published, funded, settled
• Request ?status=published and assert only the published invoice is returned
• Request ?status=funded and assert only the funded invoice is returned
• Request with no status param and assert all four are returned
• Request ?status=invalid and assert 422 with a validation error

Acceptance Criteria

• ?status=published returns only published invoices
• ?status=funded returns only funded invoices
• No status param returns all invoices
• Invalid status value returns 422

ETA: 12 hours

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