Issue #59 — Audit logging middleware

[GoSTEAN]

Title: [Middleware] Implement audit log for all admin and fund-moving actions

Tags: backend, middleware, security, feature

Description:
Record an immutable audit trail of all sensitive actions for compliance and incident response.

Acceptance Criteria:

• audit-log.middleware.ts attaches to all /admin/*, /wallet/withdraw, /disputes/* routes.
• Logs: userId, ip, method, path, requestBody (sanitised — no passwords/tokens), responseStatus, timestamp.
• Records stored in audit_logs DB table (append-only, no DELETE route).
• GET /admin/audit-logs (admin) — paginated with filters by userId, action, date range.
• Integration test: admin action → audit log entry created.

Files: src/middleware/audit-log.middleware.ts, src/routes/admin.routes.ts

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