Document mutex usage

Always document mutex fields and their purpose, ensure proper locking/unlocking patterns, and avoid redundant locks when underlying structures are already thread-safe. When using mutexes, clearly indicate what data they protect and why the synchronization is necessary.

Add comments for mutex fields explaining their purpose:

// sendfilesMutex protects concurrent access to sendfiles slice
sendfilesMutex sync.RWMutex

Avoid redundant locking when underlying structures already provide thread safety:

// The mu mutex in the Middleware struct might be redundant for operations 
// on the Session since Session struct itself already ensures thread-safety

Use proper defer patterns to prevent race conditions, even if slightly slower:

mutex.Lock()
defer mutex.Unlock() // Prevents race conditions on shared data access
return c.Status(fiber.StatusOK).JSON(data)

Consider performance implications when choosing between sync.Map and regular maps with RWMutex, but prioritize correctness and clarity over micro-optimizations.