Add dataset change tracking and latest-update summary to landing page

[elainefan331]

Description:
Add dataset-level change tracking to the NeuroJSON incremental sync pipeline so each sync run can record which datasets were added, updated, or deleted.

Tasks:

• Add a dataset_changes table to track dataset-level added, updated, and deleted events per sync run.
• Update the incremental sync flow to detect and record dataset changes, linked to stats_history.
• Add a backend endpoint to return the latest sync run with actual dataset changes.
• Show a “Latest Update” summary on the landing page.

Acceptance Criteria:

• Dataset-level added, updated, and deleted changes are recorded for incremental sync runs.
• Each dataset change is linked to the correct stats_history record.
• firstSync() does not create dataset change records.
• Dataset updates/deletions and their change-log records are committed consistently.
• The latest-update API returns the most recent sync run that contains actual dataset changes.
• The landing page displays the latest update time and a summary of added, updated, and deleted datasets.
• Existing sync and search behavior continues to work as before

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