fix(s3): fix path prefix not applied during segment cleanup (#4799)

* fix(s3): scope segment cleanup to path prefix, fix slice panic, batch deletes

When a path prefix is configured for S3 storage, uploads correctly
write to <prefix>/hls/... but cleanup lists objects from the bucket
root without the prefix. This causes a panic when the resulting
segment list is empty, due to unsafe slicing with a negative index.

- Centralize prefix construction in remoteHLSPrefix() so Save(),
  retrieveAllVideoSegments(), and rewritePlaylistLocations() all
  share the same logic. remoteHLSListingPrefix() adds a trailing
  slash for directory-scoped S3 listing.
- Use strings.Trim to normalize both leading and trailing slashes
  from the configured path prefix.
- Replace ListObjects with ListObjectsPages to handle buckets with
  more than 1000 segments.
- Batch DeleteObjects calls in chunks of 1000 to respect the S3
  API limit, and log per-object errors from the response.
- Guard getDeletableVideoSegmentsWithOffset against empty result
  sets to prevent the slice bounds panic.

Fixes #4784

* fix(s3): handle empty hlsPrefix in rewritePlaylistLocations

LocalStorage calls rewritePlaylistLocations with an empty prefix.
The previous refactor removed the fallback to "/hls", causing
double slashes and missing path segments in playlist URLs.

Default hlsPrefix to "hls" when empty to preserve existing behavior
for local storage setups with a custom video serving endpoint.

core/storageproviders/rewriteLocalPlaylist.go

@@ -13,7 +13,7 @@ import (
 )
 
 // rewritePlaylistLocations will take a local playlist and rewrite it to have absolute URLs to a specified location.
-func rewritePlaylistLocations(localFilePath, remoteServingEndpoint, pathPrefix string) error {
+func rewritePlaylistLocations(localFilePath, remoteServingEndpoint, hlsPrefix string) error {
 	f, err := os.Open(localFilePath) // nolint
 	if err != nil {
 		log.Fatalln(err)
@@ -24,15 +24,12 @@ func rewritePlaylistLocations(localFilePath, remoteServingEndpoint, pathPrefix s
 		log.Warnln(err)
 	}
 
+	if hlsPrefix == "" {
+		hlsPrefix = "hls"
+	}
+
 	for _, item := range p.Variants {
-		// Determine the final path to this playlist.
-		var finalPath string
-		if pathPrefix != "" {
-			finalPath = filepath.Join(pathPrefix, "/hls")
-		} else {
-			finalPath = "/hls"
-		}
-		item.URI = remoteServingEndpoint + filepath.Join(finalPath, item.URI)
+		item.URI = remoteServingEndpoint + "/" + hlsPrefix + "/" + item.URI
 	}
 
 	publicPath := filepath.Join(config.HLSStoragePath, filepath.Base(localFilePath))