fix: restore legacy usage backups safely

[ssfun]

Summary

• force Docker WebDAV restore to use the explicit legacy import compatibility flag during the transition
• allocate model price rule revisions from the highest retained history version instead of the active version
• add a manifest-backed import regression test for retained model price history and update documentation

Validation

• PYTHONPYCACHEPREFIX=/private/tmp/cliproxy-backup-import-pycache bash scripts/validation/repo.sh
• Core v7.2.95: complete scripts/validation/core.sh on a fresh generated checkout
• regression: TestUsageImportRestoresModelPriceRuleWhenOnlyNewerHistoryRemains
• full embeddedusage tests and server build