Update earliest_available_slot progressively as backfill imports durable block and data-column history

[satushh]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

• During checkpoint-sync backfill, the node kept advertising the checkpoint-era earliest_available_slot even as older blocks and data-column sidecars were durably imported.
• This PR lowers both the persisted (db) and advertised (p2p custody) earliest_available_slot to the importer-returned BackfillStatus.LowSlot after each committed batch, so Status v2 progresses during a long backfill and survives restarts; updates are attempted independently, never move the value forward, only log failures (a failed update cannot re-import or fail a committed batch), and remain a no-op pre-Fulu.
• Conservative guards skip lowering when the custody group count changed since backfill started or the advertised value was already raised above the backfill origin — re-establishing column coverage for an increased custody scope is left as future work (partial progress on Post-Fulu Backfill #15982).

Which issue(s) does this PR fix?

Fixes #

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).