feat(session): broadcast participant_joined event via gRPC for real-time UI updates

Backend changes (session-coordinator):
- Add PublishParticipantJoined method to JoinSessionMessageRouterClient interface
- Implement PublishParticipantJoined in MessageRouterClient to broadcast events
- Call PublishParticipantJoined in join_session.go after participant joins
- Add detailed logging for debugging event broadcast

Android changes (service-party-android):
- Add detailed logging in TssRepository for session event handling
- Add detailed logging in MainViewModel for participant_joined processing
- Log activeSession state, event matching, and participant updates

This enables the initiator's waiting screen to receive real-time updates
when participants join the session, matching the expected behavior.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/data/repository/TssRepository.kt

@@ -148,27 +148,44 @@ class TssRepository @Inject constructor(
      */
     private fun startSessionEventSubscription() {
         sessionEventJob?.cancel()
+        android.util.Log.d("TssRepository", "Starting session event subscription for partyId: $partyId")
         sessionEventJob = CoroutineScope(Dispatchers.IO).launch {
             grpcClient.subscribeSessionEvents(partyId).collect { event ->
-                android.util.Log.d("TssRepository", "Session event received: ${event.eventType} for session ${event.sessionId}")
+                android.util.Log.d("TssRepository", "=== Session event received ===")
+                android.util.Log.d("TssRepository", "  eventType: ${event.eventType}")
+                android.util.Log.d("TssRepository", "  sessionId: ${event.sessionId}")
+                android.util.Log.d("TssRepository", "  selectedParties: ${event.selectedParties}")
 
                 // Check if this event is for our active session
                 val activeSession = _currentSession.value
+                android.util.Log.d("TssRepository", "  activeSession: ${activeSession?.sessionId ?: "null"}")
+
                 if (activeSession != null && event.sessionId == activeSession.sessionId) {
+                    android.util.Log.d("TssRepository", "  → Event matches active session!")
                     when (event.eventType) {
                         "session_started" -> {
-                            android.util.Log.d("TssRepository", "Session started event for our session, triggering keygen")
+                            android.util.Log.d("TssRepository", "  → Processing session_started event")
                             // Notify callback
                             sessionEventCallback?.invoke(event)
                         }
                         "party_joined", "participant_joined" -> {
-                            android.util.Log.d("TssRepository", "Party joined our session")
+                            android.util.Log.d("TssRepository", "  → Processing participant_joined event")
                             sessionEventCallback?.invoke(event)
                         }
                         "all_joined" -> {
-                            android.util.Log.d("TssRepository", "All parties joined our session")
+                            android.util.Log.d("TssRepository", "  → Processing all_joined event")
                             sessionEventCallback?.invoke(event)
                         }
+                        else -> {
+                            android.util.Log.d("TssRepository", "  → Unknown event type: ${event.eventType}")
+                        }
+                    }
+                } else {
+                    android.util.Log.d("TssRepository", "  → Event does NOT match active session (ignored)")
+                    if (activeSession == null) {
+                        android.util.Log.d("TssRepository", "    Reason: activeSession is null")
+                    } else {
+                        android.util.Log.d("TssRepository", "    Reason: sessionId mismatch (event: ${event.sessionId}, active: ${activeSession.sessionId})")
                     }
                 }
             }

backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt

@@ -262,8 +262,14 @@ class MainViewModel @Inject constructor(
      * - CoSign joiner (参与签名)
      */
     private fun setupSessionEventCallback() {
+        android.util.Log.d("MainViewModel", "Setting up session event callback")
         repository.setSessionEventCallback { event ->
-            android.util.Log.d("MainViewModel", "Session event: ${event.eventType} for session ${event.sessionId}")
+            android.util.Log.d("MainViewModel", "=== MainViewModel received session event ===")
+            android.util.Log.d("MainViewModel", "  eventType: ${event.eventType}")
+            android.util.Log.d("MainViewModel", "  sessionId: ${event.sessionId}")
+            android.util.Log.d("MainViewModel", "  _currentSessionId: ${_currentSessionId.value}")
+            android.util.Log.d("MainViewModel", "  pendingJoinKeygenInfo?.sessionId: ${pendingJoinKeygenInfo?.sessionId}")
+            android.util.Log.d("MainViewModel", "  pendingJoinSignInfo?.sessionId: ${pendingJoinSignInfo?.sessionId}")
 
             when (event.eventType) {
                 "session_started" -> {
@@ -296,18 +302,25 @@ class MainViewModel @Inject constructor(
                     }
                 }
                 "party_joined", "participant_joined" -> {
+                    android.util.Log.d("MainViewModel", "Processing participant_joined event...")
+
                     // Update participant count for initiator's CreateWallet screen
                     val currentSessionId = _currentSessionId.value
+                    android.util.Log.d("MainViewModel", "  Checking for initiator: currentSessionId=$currentSessionId, eventSessionId=${event.sessionId}")
                     if (currentSessionId != null && event.sessionId == currentSessionId) {
+                        android.util.Log.d("MainViewModel", "  → Matched initiator session! Updating _sessionParticipants")
                         _sessionParticipants.update { current ->
                             val newParticipant = "参与方 ${current.size + 1}"
+                            android.util.Log.d("MainViewModel", "  → Adding participant: $newParticipant, total now: ${current.size + 1}")
                             current + newParticipant
                         }
                     }
 
                     // Update participant count for keygen joiner's JoinKeygen screen
                     val joinKeygenInfo = pendingJoinKeygenInfo
+                    android.util.Log.d("MainViewModel", "  Checking for joiner: joinKeygenInfo?.sessionId=${joinKeygenInfo?.sessionId}")
                     if (joinKeygenInfo != null && event.sessionId == joinKeygenInfo.sessionId) {
+                        android.util.Log.d("MainViewModel", "  → Matched joiner session! Updating _joinKeygenParticipants")
                         _joinKeygenParticipants.update { current ->
                             val newParticipant = "参与方 ${current.size + 1}"
                             current + newParticipant
@@ -317,6 +330,7 @@ class MainViewModel @Inject constructor(
                     // Update participant count for sign joiner's CoSign screen
                     val joinSignInfo = pendingJoinSignInfo
                     if (joinSignInfo != null && event.sessionId == joinSignInfo.sessionId) {
+                        android.util.Log.d("MainViewModel", "  → Matched sign joiner session! Updating _coSignParticipants")
                         _coSignParticipants.update { current ->
                             val newParticipant = "参与方 ${current.size + 1}"
                             current + newParticipant

backend/mpc-system/services/session-coordinator/adapters/output/grpc/message_router_client.go

@@ -171,3 +171,32 @@ func (c *MessageRouterClient) PublishSessionStarted(
 
 	return c.PublishSessionEvent(ctx, event)
 }
+
+// PublishParticipantJoined publishes a participant_joined event to all parties in the session
+// This notifies the initiator and other participants that a new party has joined
+func (c *MessageRouterClient) PublishParticipantJoined(
+	ctx context.Context,
+	sessionID string,
+	partyID string,
+	selectedParties []string,
+	joinedAt int64,
+) error {
+	logger.Info("Publishing participant_joined event to Message Router",
+		zap.String("session_id", sessionID),
+		zap.String("joined_party_id", partyID),
+		zap.Strings("notify_parties", selectedParties),
+		zap.Int64("joined_at", joinedAt))
+
+	event := &router.SessionEvent{
+		EventId:         uuid.New().String(),
+		EventType:       "participant_joined",
+		SessionId:       sessionID,
+		SelectedParties: selectedParties,
+		CreatedAt:       joinedAt,
+		// Note: We could add a custom field for the joined party ID, but for now
+		// the event itself indicates someone joined. The initiator can refresh
+		// their participant list via API if needed.
+	}
+
+	return c.PublishSessionEvent(ctx, event)
+}

backend/mpc-system/services/session-coordinator/application/use_cases/join_session.go

@@ -32,6 +32,16 @@ type JoinSessionMessageRouterClient interface {
 		joinTokens map[string]string,
 		startedAt int64,
 	) error
+
+	// PublishParticipantJoined broadcasts a participant_joined event to all parties in the session
+	// This allows the initiator's waiting screen to update in real-time when participants join
+	PublishParticipantJoined(
+		ctx context.Context,
+		sessionID string,
+		partyID string,
+		selectedParties []string,
+		joinedAt int64,
+	) error
 }
 
 // JoinSessionUseCase implements the join session use case
@@ -271,19 +281,54 @@ func (uc *JoinSessionUseCase) executeWithRetry(
 		return nil, err
 	}
 
-	// 9. Publish participant joined event
+	// 9. Publish participant joined event to internal message broker
+	joinedAt := time.Now().UnixMilli()
 	event := output.ParticipantJoinedEvent{
 		SessionID: session.ID.String(),
 		PartyID:   inputData.PartyID,
-		JoinedAt:  time.Now().UnixMilli(),
+		JoinedAt:  joinedAt,
 	}
 	if err := uc.eventPublisher.PublishEvent(ctx, output.TopicParticipantJoined, event); err != nil {
-		logger.Error("failed to publish participant joined event",
+		logger.Error("failed to publish participant joined event to internal broker",
 			zap.String("session_id", session.ID.String()),
 			zap.String("party_id", inputData.PartyID),
 			zap.Error(err))
 	}
 
+	// 9.1 Publish participant joined event via gRPC to message-router (for real-time UI updates)
+	// This notifies the initiator and other participants that a new party has joined
+	if uc.messageRouterClient != nil {
+		// Get all party IDs in the session to notify them
+		allPartyIDs := session.GetPartyIDs()
+		logger.Info("Broadcasting participant_joined event via gRPC",
+			zap.String("session_id", session.ID.String()),
+			zap.String("joined_party_id", inputData.PartyID),
+			zap.Strings("notify_parties", allPartyIDs),
+			zap.Int("total_participants", len(session.Participants)))
+
+		if err := uc.messageRouterClient.PublishParticipantJoined(
+			ctx,
+			session.ID.String(),
+			inputData.PartyID,
+			allPartyIDs,
+			joinedAt,
+		); err != nil {
+			logger.Error("failed to publish participant joined event to message router",
+				zap.String("session_id", session.ID.String()),
+				zap.String("party_id", inputData.PartyID),
+				zap.Error(err))
+		} else {
+			logger.Info("Successfully published participant_joined event to message router",
+				zap.String("session_id", session.ID.String()),
+				zap.String("joined_party_id", inputData.PartyID),
+				zap.Int("notify_count", len(allPartyIDs)))
+		}
+	} else {
+		logger.Warn("messageRouterClient is nil, cannot broadcast participant_joined event",
+			zap.String("session_id", session.ID.String()),
+			zap.String("party_id", inputData.PartyID))
+	}
+
 	// 10. Build response with other parties info
 	otherParties := session.GetOtherParties(partyID)
 	partyInfos := make([]input.PartyInfo, len(otherParties))