feat: add fallback backup manifests and recovery telemetry events

[jsugg]

Summary

• embed a deterministic manifest into tar fallback backups with profile/include/exclude/bytes/file_count/hash/backend metadata
• validate fallback manifests during backup verification before restore flow proceeds
• emit structured recovery events for backup create/plan/verify/prune/restore with consistent scalar fields
• extend recovery unit tests to cover manifest validation and telemetry event payload presence