fix(context): use parent context instead of Background() to allow proper cancellation

- Fixed delegate party event handler to use parent context with timeout
- Fixed message acknowledgment to use parent context
- Prevents orphan goroutines when session fails or party exits
- Resolves system crash after delegate party failure

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

@@ -247,7 +247,9 @@ 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" {
@@ -264,7 +266,7 @@ 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),
@@ -285,7 +287,7 @@ func createSessionEventHandler(
 							zap.Int("share_size", len(result.ShareForUser)))
 
 						if err := messageRouter.SubmitDelegateShare(
-							ctx,
+							participateCtx,
 							event.SessionId,
 							partyID,
 							result.ShareForUser,
@@ -316,7 +318,7 @@ func createSessionEventHandler(
 						// This will be passed through the session event or retrieved separately
 					}
 
-					result, err := participateSigningUC.Execute(ctx, input)
+					result, err := participateSigningUC.Execute(participateCtx, input)
 					if err != nil {
 						logger.Error("Signing participation failed",
 							zap.Error(err),

backend/mpc-system/services/server-party/adapters/output/grpc/message_router_client.go

@@ -181,7 +181,8 @@ func (c *MessageRouterClient) SubscribeMessages(
 
 					// Send acknowledgment for the received message
 					go func(messageID, sessionIDStr, pID string) {
-						ackCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+						// Use parent context to allow proper cancellation
+						ackCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
 						defer cancel()
 						sid, _ := uuid.Parse(sessionIDStr)
 						if err := c.AcknowledgeMessage(ackCtx, messageID, sid, pID, true, ""); err != nil {