fix(test): update JWT secret to match .env configuration

Fixed JWT secret in test_create_session.go to use the same secret key
as configured in .env file, resolving 401 Unauthorized errors during
keygen session creation tests.

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

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

backend/mpc-system/test_create_session.go

@@ -43,7 +43,7 @@ func main() {
 	// Get JWT secret from environment or use test value
 	jwtSecret := os.Getenv("JWT_SECRET_KEY")
 	if jwtSecret == "" {
-		jwtSecret = "test-jwt-secret-key-please-change-in-production"
+		jwtSecret = "change_this_jwt_secret_key_to_random_value_min_32_chars"
 	}
 
 	// Generate access token