Service degradation via unbounded path mutex retention

Uncontrolled resource consumption in SiYuan before 3.8.2 allows remote attackers to degrade availability via unique request paths. The model.ControlConcurrency middleware inserts a mutex into the global requesting map for each new path before routing, but never evicts it after completion. No authentication or valid route is required; repeated unique 404 paths permanently grow process state, increasing memory pressure and synchronization overhead without reading or writing workspace data.