feat: Implement centralized audit logging admin interface (refs #10)

- Add audit_logs table with categories, severities, and indexes
- Create AuditLogService and AuditLogRepository
- Add REST API endpoints for viewing and exporting logs
- Wire audit logging into auth, vehicles, admin, and backup features
- Add desktop AdminLogsPage with filters and CSV export
- Add mobile AdminLogsMobileScreen with card layout
- Implement 90-day retention cleanup job
- Remove old AuditLogPanel from AdminCatalogPage

Security fixes:
- Escape LIKE special characters to prevent pattern injection
- Limit CSV export to 5000 records to prevent memory exhaustion
- Add truncation warning headers for large exports

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/audit-log/audit-log.instance.ts

@@ -0,0 +1,14 @@
+/**
+ * @ai-summary Singleton audit log service instance
+ * @ai-context Provides centralized audit logging across all features
+ */
+
+import { pool } from '../../core/config/database';
+import { AuditLogRepository } from './data/audit-log.repository';
+import { AuditLogService } from './domain/audit-log.service';
+
+// Create singleton repository and service instances
+const repository = new AuditLogRepository(pool);
+export const auditLogService = new AuditLogService(repository);
+
+export default auditLogService;