fix(azcopy): keep archived backups out of latest-backup checks

[SakuraIsayeki]

Archived historical Azure manifests currently prevent Backmaster from discovering a newer backup. The exporter downloads every manifest before selecting the latest epoch, and the same scan also breaks serial allocation and retention.

This change adds an online catalogue at catalogue/<created_epoch>/<backup_name>.json. Publication uploads the backup manifest after its payload, then the catalogue record in Hot storage. Freshness reads only the newest catalogue record; serial allocation and retention use listings without downloading archived manifests. Failed catalogue publication preserves the existing ready-stage retry flow.

The exporter now propagates listing failures, distinguishes failed downloads from invalid JSON, validates catalogue names/epochs, rejects conflicting backup-name reuse, and prevents stale catalogue records from counting toward minimum redundancy. Decimal serial allocation also handles 008/009 correctly.

Migration is explicit: upgrade all writers, exclude catalogue/ from lifecycle archival/deletion, then import the newest verified readable backup with catalogue-import NAME or run one forced backup with a unique name. Legacy unindexed backups are preserved with a retention warning; they are never silently deleted or scanned for freshness. The exporter reference, example configuration, and troubleshooting guide document rollout and failure recovery.

Validation:

• All seven repository test scripts passed, including Debian packaging.
• ShellCheck 0.11.0 and Bash syntax checks passed.
• Regression coverage includes archived old manifests, latest-only reads, legacy import, listing failures with partial output, missing/corrupt newest records, upload/removal failures, stale catalogue entries, serial 009, and name collisions.
• A core integration test verifies catalogue failure retains the ready stage, retry avoids a second prepare, and the next fresh run skips.
• Azure operations were exercised with the repository's AzCopy mock; no live Azure deployment was performed.