feat(09-01): add scan control events and cancelled metrics field

- Add LibraryScanCancelled, LibraryScanPaused, LibraryScanResumed events
- Regenerate frontend/src/events.ts via go generate
- Add Cancelled bool field to ScanMetrics struct

backend/library/metrics.go

@@ -50,6 +50,9 @@ type ScanMetrics struct {
 	Skipped int64 `json:"skipped"`
 	Removed int64 `json:"removed"`
 
+	// Cancelled is true when the scan was stopped via CancelScan.
+	Cancelled bool `json:"cancelled"`
+
 	// Non-fatal issues encountered during scanning.
 	Warnings []ScanWarning `json:"warnings"`
 }