[BE-10] Add request logging middleware and integrate the dormant AuditLog

[Leothosine]

Problem

Two observability gaps exist:

1. There is no HTTP request logging middleware - no log of incoming requests, response status codes, or latency. Production debugging is blind.
2. src/modules/admin/admin.service.ts:283 defines a logAction() method with a complete AuditLog entity behind it, but logAction() is never called from any controller or service. Every admin mutation is untracked.

Proposed Solution

Request logging:

• Create src/common/middleware/logger.middleware.ts that generates a UUID correlationId per request
• Log method, url, correlationId on arrival and statusCode + duration on response finish
• Redact the Authorization header from logs

AuditLog integration:

• Call logAction() from: createUser, updateUserStatus, updateUserRole, deleteUser in AdminService
• Ensure GET /api/admin/audit-logs supports filtering by action, startDate, endDate with pagination

Acceptance Criteria

• Every HTTP request produces a structured log line with correlationId, method, URL, status, and duration
• Authorization header value is redacted in logs
• All admin mutation endpoints create an AuditLog record
• GET /api/admin/audit-logs returns paginated results filterable by action and date range
• correlationId appears in both the request log and service-level logs within the same request