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

## 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 `-backup-` 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).

Start at the restore path that resolves a requested backupID after the Klio catalog lookup, then trace the object-store read path and configuration gate. Use the verification checklist to define done: read-only restoration of single- and multi-tablespace Barman Cloud tar backups across the listed compression variants, plus integration/e2e coverage and disabled-mode call-volume and write-safety checks.

Written by the indexing model from the issue text.