Add integration test for the admin invoice reject endpoint storing the rejection reason and notifying the seller

[Chucks1093]

Summary

When an admin rejects an invoice, the rejection reason should be stored on the invoice record and a notification should be created for the seller. An integration test should confirm both side effects occur and that the invoice status transitions to rejected.

Scope

• Seed a pending_review invoice owned by seller A
• Call the admin reject endpoint with a rejection reason string
• Assert the invoice status is rejected
• Assert the rejection_reason field on the invoice record matches the submitted reason
• Assert a notification exists for seller A's wallet with the rejection reason in the message

Acceptance Criteria

• Invoice status transitions to rejected
• rejection_reason persisted on the invoice record
• Notification created for the seller wallet
• Non-admin caller receives 403
• Rejecting an already-rejected invoice returns 409

ETA: 12 hours

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