Allow incremental ingestion without reprocessing entire history

[hotoke-no-Kami]

Overview

Enhance the ingestion workflow to support incremental data processing so that running ingestion does not reprocess the entire ledger history every time.

Acceptance Criteria

• Tracks last processed ledger: System persists the last successfully processed ledger. Upon subsequent ingestion runs, only new ledgers are processed.
• Idempotent execution: Running the ingestion multiple times without new data does not cause duplicate processing, errors, or side effects.
• CLI command available: A command-line interface is available to trigger incremental ingestion, e.g., python -m astroml.ingestion.incremental (or similar, per project conventions).

Notes

• Ensure the marker for the last processed ledger is robust and persists across restarts/crashes.
• Handle initialization (if no previous state is found) by starting from the configured start ledger or from the earliest available in the database.
• Update README with usage instructions for the new CLI.