fix: make slug package thread safe

internal/session/slug/slug.go

@@ -1,11 +1,14 @@
 package slug
 
+import "sync"
+
 type Slug interface {
 	String() string
 	Set(slug string)
 }
 
 type slug struct {
+	mu   sync.RWMutex
 	slug string
 }
 
@@ -16,9 +19,13 @@ func New() Slug {
 }
 
 func (s *slug) String() string {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
 	return s.slug
 }
 
 func (s *slug) Set(slug string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
 	s.slug = slug
 }