fix(server-party): use parent context in event handler for proper cancellation

- Fixed server-party event handler to use parent context with timeout
- Prevents orphan goroutines when session fails or party exits
- Consistent with server-party-api fix

backend/mpc-system/services/server-party/cmd/server/main.go

@@ -568,7 +568,9 @@ func createSessionEventHandler(
 
 		// Automatically participate based on session type
 		go func() {
-			ctx := context.Background()
+			// Use parent context to allow proper cancellation
+			participateCtx, cancel := context.WithTimeout(ctx, 5*time.Minute)
+			defer cancel()
 
 			// Determine session type from event
 			if event.EventType == "session_created" {
@@ -585,7 +587,7 @@ func createSessionEventHandler(
 						JoinToken: joinToken,
 					}
 
-					result, err := participateKeygenUC.Execute(ctx, input)
+					result, err := participateKeygenUC.Execute(participateCtx, input)
 					if err != nil {
 						logger.Error("Keygen participation failed",
 							zap.Error(err),
@@ -617,7 +619,7 @@ func createSessionEventHandler(
 							zap.Int32("party_index", event.DelegateUserShare.PartyIndex))
 					}
 
-					result, err := participateSigningUC.Execute(ctx, input)
+					result, err := participateSigningUC.Execute(participateCtx, input)
 					if err != nil {
 						logger.Error("Signing participation failed",
 							zap.Error(err),