Backup restore: support reading base backups from a Barman Cloud object-store layout as a fallback (read-only, tar-based)

[gbartolini]

Problem

Klio has no way to restore a base backup produced by the Barman Cloud plugin prior to migration, unless that backup is deliberately imported into Kopia. When a requested backupID isn't found in Klio's own catalog, there's currently no way to know whether it exists in a Barman Cloud tar-mode layout in the same object store, nor any way to retrieve it.

Requirements

• Read-only: no re-encoding of Barman Cloud backups into Kopia, no retention, pinning, or write of any kind against them.
• No dependency on the barman-cloud plugin being installed or running.
• Must handle Barman Cloud's tar-mode backup layout, including multi-tablespace backups, and all its supported compression variants (none, gzip, bzip2, snappy).
• Backup ID resolution between Klio's <cluster>-backup-<timestamp> convention and Barman Cloud's own backup-id format must be unambiguous in both directions.
• Behaviour must be configuration-gated: fully inert when disabled, with no extra object-store calls and no change to existing behaviour.

Verification

• A restore request for a backupID absent from Klio's native catalog successfully resolves and restores from a matching Barman Cloud tar-mode backup, feature enabled.
• Multi-tablespace backups restore to the correct locations.
• All three compression variants restore correctly.
• With the feature disabled, behaviour and object-store call volume are identical to today.
• No write, retention, or pin operation is ever issued against the Barman Cloud objects, confirmed by test.
• Covered by an integration/e2e test against a real Barman Cloud tar backup (single- and multi-tablespace cases).

Open questions

• Confirm exact Barman Cloud backup-id format and timezone semantics.
• Confirm the mapping between a Klio backupID and its corresponding Barman Cloud backup-id is unambiguous in both directions.

Out of scope

• Any write path back into Klio's own catalog.
• WAL retrieval past the restored base backup (sibling WAL fallback ticket).