s3: fix pathCache race condition

Make sure pathCache is properly locked for concurrent access.

Add RWMutex to the PublishedStorage struct:
- Cache initialization
  Read-lock to test for nil, then write-lock with a second nil check before populating
- Cache reads
  RLock/RUnlock, allowing concurrent readers
- Cache writes / deletes
  Lock/Unlock

api/s3.go

@@ -14,7 +14,8 @@ import (
 // @Router /api/s3 [get]
 func apiS3List(c *gin.Context) {
 	keys := []string{}
-	for k := range context.Config().S3PublishRoots {
+	s3Roots := context.Config().S3PublishRoots
+	for k := range s3Roots {
 		keys = append(keys, k)
 	}
 	c.JSON(200, keys)