Fix data races across session/forwarder/registry, resolve POST/PUT hang, harden port allocation, add frontendURL env (#157)

Reviewed-on: #157
Co-authored-by: Bagas <bagas@fossy.my.id>
Co-committed-by: Bagas <bagas@fossy.my.id>

internal/registry/registry.go

@@ -94,13 +94,13 @@ func (r *registry) Update(user string, oldKey, newKey Key) error {
 		return ErrInvalidSlug
 	}
 
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
 	if _, exists := r.slugIndex[newKey]; exists && newKey != oldKey {
 		return ErrSlugInUse
 	}
 
-	r.mu.Lock()
-	defer r.mu.Unlock()
-
 	client, ok := r.byUser[user][oldKey]
 	if !ok {
 		return ErrSessionNotFound