Report upload error status in metrics

[biglittlebigben]

Summary

• Label the pipeline_uploads failure counter with the HTTP status class (4xx, 5xx, or internal) instead of a single failure value, so user errors (bad creds, missing bucket, access denied) can be distinguished from infrastructure issues.
• Extract the status code from storage errors via errors.As(&storage.ErrorWithStatusCode{}).
• Switch ErrUploadFailed to %w so the underlying storage error is preserved through the wrap.

Dependencies

Depends on livekit/storage#30 (introduces storage.ErrorWithStatusCode). go.mod currently has a local replace pointing at ../storage so the branches can be reviewed together; this will be dropped once the storage PR merges.

Test plan

• go test ./pkg/pipeline/sink/uploader/... — new TestUploadErrorHasStatusCode covers 4xx and 5xx S3 responses.
• Verify the pipeline_uploads Prometheus metric shows status={success,4xx,5xx,internal} in a real egress run.

🤖 Generated with Claude Code

[github-actions]

AV-sync stats summary: view in run #28048493260