fix(account): exclude failed sessions when looking up sign session by invite code

When multiple sign sessions share the same invite code (due to retries),
the query now:
1. Excludes failed sessions (status != 'failed')
2. Orders by created_at DESC to get the most recent session
3. Limits to 1 result

This prevents participants from seeing an old failed session's status
when they look up the invite code.

🤖 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

@@ -691,7 +691,9 @@ func (h *CoManagedHTTPHandler) GetSignSessionByInviteCode(c *gin.Context) {
 		SELECT id, COALESCE(wallet_name, ''), COALESCE(keygen_session_id::text, ''),
 		       threshold_n, threshold_t, status, expires_at, COALESCE(message_hash, '')
 		FROM mpc_sessions
-		WHERE invite_code = $1 AND session_type = 'sign'
+		WHERE invite_code = $1 AND session_type = 'sign' AND status != 'failed'
+		ORDER BY created_at DESC
+		LIMIT 1
 	`, inviteCode).Scan(&sessionID, &walletName, &keygenSessionID, &thresholdN, &thresholdT, &status, &expiresAt, &messageHash)
 
 	if err != nil {