* 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.
