Get Security Scan

Task #326: surfaces the security watch (scripts/security_scan.py / services/security_scan_service.py) in the admin panel, instead of it only ever existing as terminal output from the external 6-hourly scheduled task. Returns the most recent scan result already persisted into xfinlab.db by the in-process APScheduler job (backend/main.py) or a prior manual run -- this is intentionally fast/read-only, it does NOT run a fresh scan on every page load.