STAC-24557: Show backup sizes in victoria-metrics list command

[viliakov]

Summary

• Add SIZE column to the victoria-metrics list command output
• Calculate total backup size by summing all S3 objects in each backup directory (with pagination support)
• Use existing output.FormatBytes() for human-readable size formatting (e.g., "977MiB")

Test plan

• Run sts-backup victoria-metrics list and verify SIZE column appears
• Verify sizes are displayed in human-readable format (B, KiB, MiB, GiB)
• Verify behavior when backups have many objects (pagination)

[viliakov]

❯ go run main.go victoria-metrics list --namespace stac-24557
Setting up port-forward to suse-observability-s3proxy:9000 in namespace stac-24557...
✅ Port-forward established on localhost:55760
Listing VictoriaMetrics backups in bucket ...

NOTE: In HA mode, backups from both instances (victoria-metrics-0 and victoria-metrics-1) are listed.
      The restore command accepts either backup to restore both instances.

NAME ({bucket}/{instance}-{created})                                                      UPDATED                  SIZE
sandbox-main-tenant-backup/stac-24557/victoria-metrics/victoria-metrics-1-latest          2026-04-08 08:35:10 UTC  1GiB
sandbox-main-tenant-backup/stac-24557/victoria-metrics/victoria-metrics-0-latest          2026-04-08 08:25:10 UTC  1GiB
sandbox-main-tenant-backup/stac-24557/victoria-metrics/victoria-metrics-0-20260408074122  2026-04-08 07:41:32 UTC  1GiB
sandbox-main-tenant-backup/stac-24557/victoria-metrics/victoria-metrics-0-20260408073312  2026-04-08 07:33:20 UTC  1GiB
sandbox-main-tenant-backup/stac-24557/victoria-metrics/victoria-metrics-0-20260408073240  2026-04-08 07:32:47 UTC  1GiB
sandbox-main-tenant-backup/stac-24557/victoria-metrics/victoria-metrics-1-20260403123500  2026-04-06 16:35:06 UTC  364MiB