[Admin] Merchant management — list, view, update, suspend

[Xaxxoo]

Overview

Full admin CRUD for merchant accounts.

Tasks

• GET /api/v1/admin/merchants — paginated with search
• GET /api/v1/admin/merchants/:id — full detail including internal fields
• PATCH /api/v1/admin/merchants/:id — update any field
• PATCH /api/v1/admin/merchants/:id/status — activate/suspend

Acceptance Criteria

• Search by email, businessName, country
• Status change triggers email notification to merchant
• All changes logged in audit trail