fix(android): refresh session event subscription when joining sign session

The session event gRPC stream may silently disconnect without triggering
onError or onCompleted callbacks. This causes session_started events to
be lost, preventing the sign process from starting.

Changes:
- Add ensureSessionEventSubscriptionActive() to refresh event subscription
- Call it in joinSignSessionViaGrpc for sign joiner
- Call it in createSignSession for sign initiator after auto-join

This ensures a fresh event stream connection before waiting for events.

🤖 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

@@ -276,6 +276,27 @@ class TssRepository @Inject constructor(
         }
     }
 
+    /**
+     * Ensure session event subscription is active
+     * Called before critical operations (like joining sign session) to ensure
+     * the event stream hasn't silently disconnected
+     */
+    private fun ensureSessionEventSubscriptionActive() {
+        // Check if the session event job is still active
+        val isActive = sessionEventJob?.isActive == true
+        android.util.Log.d("TssRepository", "Checking session event subscription: isActive=$isActive")
+
+        if (!isActive) {
+            android.util.Log.w("TssRepository", "Session event subscription is not active, restarting...")
+            startSessionEventSubscription()
+        } else {
+            // Even if the job is "active", the gRPC stream may have silently disconnected
+            // Force a restart to ensure we have a fresh connection
+            android.util.Log.d("TssRepository", "Refreshing session event subscription to ensure fresh connection")
+            startSessionEventSubscription()
+        }
+    }
+
     /**
      * Set session event callback (called by ViewModel)
      */
@@ -1143,6 +1164,10 @@ class TssRepository @Inject constructor(
                 // Start message subscription (matching Electron's prepareForSign)
                 startMessageRouting(sessionId, myPartyIndex)
 
+                // CRITICAL: Ensure session event subscription is active
+                // The event stream may have silently disconnected, so refresh it
+                ensureSessionEventSubscriptionActive()
+
                 android.util.Log.d("TssRepository", "Sign session state set, waiting for session_started event or in_progress status")
 
                 Result.success(JoinSignViaGrpcResult(
@@ -2049,6 +2074,10 @@ class TssRepository @Inject constructor(
                         // Step 5: Start message routing (prepareForSign) BEFORE sign starts
                         startMessageRouting(sessionId, myPartyIndex)
 
+                        // CRITICAL: Ensure session event subscription is active for sign initiator
+                        // The event stream may have silently disconnected
+                        ensureSessionEventSubscriptionActive()
+
                         // Step 6: Check if session already in_progress
                         if (joinData.sessionStatus == "in_progress") {
                             android.util.Log.d("TssRepository", "[CO-SIGN] Session already in_progress, will trigger sign immediately")