feat: add WebSocket message types for scan progress

Add new message type constants for real-time scan progress updates:
- MessageTypeScanProgress: broadcast progress during scanning
- MessageTypeScanComplete: notify when scan completes
- MessageTypeScanError: report scan errors

These enable frontend to receive live scan updates instead of polling.

internal/sync/websocket.go

@@ -17,6 +17,9 @@ const (
 	MessageTypeSyncComplete     = "sync_complete"
 	MessageTypeHeartbeat        = "heartbeat"
 	MessageTypeInitial          = "initial_state"
+	MessageTypeScanProgress     = "scan_progress"
+	MessageTypeScanComplete     = "scan_complete"
+	MessageTypeScanError        = "scan_error"
 )
 
 // BroadcastMessage represents a message to broadcast to connected clients