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/CLAUDE.md

@@ -7,6 +7,7 @@ Feature capsule directory. Each feature is 100% self-contained with api/, domain
 | Directory | What | When to read |
 | --------- | ---- | ------------ |
 | `admin/` | Admin role management, catalog CRUD | Admin functionality, user oversight |
+| `audit-log/` | Centralized audit logging | Cross-feature event logging, admin logs UI |
 | `auth/` | Authentication endpoints | Login, logout, session management |
 | `backup/` | Database backup and restore | Backup jobs, data export/import |
 | `documents/` | Document storage and management | File uploads, document handling |