refactor(account): use statusResp.Participants for parties in GetSignSessionByInviteCode

Same approach as keygen - get participants from session coordinator
instead of querying database directly.

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

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

backend/mpc-system/services/account/adapters/input/http/co_managed_handler.go

@@ -685,25 +685,14 @@ func (h *CoManagedHTTPHandler) GetSignSessionByInviteCode(c *gin.Context) {
 		}
 	}
 
-	// Query participants for this session
+	// Build parties from statusResp.Participants (same as keygen)
 	var parties []gin.H
-	partiesRows, err := h.db.QueryContext(ctx, `
-		SELECT party_id, party_index
-		FROM participants
-		WHERE session_id = $1
-		ORDER BY party_index
-	`, sessionID)
-	if err == nil {
-		defer partiesRows.Close()
-		for partiesRows.Next() {
-			var partyID string
-			var partyIndex int
-			if err := partiesRows.Scan(&partyID, &partyIndex); err == nil {
-				parties = append(parties, gin.H{
-					"party_id":    partyID,
-					"party_index": partyIndex,
-				})
-			}
+	if len(statusResp.Participants) > 0 {
+		for _, p := range statusResp.Participants {
+			parties = append(parties, gin.H{
+				"party_id":    p.PartyID,
+				"party_index": p.PartyIndex,
+			})
 		}
 	}