[kelvinokwudili52-stack]

Summary

When the indexer resumes polling after an outage or downtime longer than Stellar RPC's retention window (~7 days), startLedger can fall behind oldestLedger. Previously, GetEventsResult only passed through events and latestLedger, leaving unrecoverable gaps untracked and risking false reconciliation breaks.

This change:

• Passes through oldestLedger from the RPC getEvents response in services/indexer/src/rpc.ts.
• Adds chain_event_gap table and migration in @ballast/db (packages/db/src/schema.ts) to store lost ledger ranges (from_ledger, to_ledger) for future data-lake backfills.
• Adds indexer_gap alert type to @ballast/observability and sets cooldown per gap range.
• Updates pollOnce in services/indexer/src/poller.ts to:
  • Detect when startLedger < oldestLedger.
  • Clamp startLedger to oldestLedger.
  • Log the lost range with structured logger.warn.
  • Fire an indexer_gap alert with fromLedger, toLedger, and oldestLedger.
  • Record the gap in chain_event_gap.
  • Update getLastIngestedLedger to consider recorded gaps so subsequent polling cycles do not re-alert on the same gap.
• Adds unit tests in services/indexer/src/poller.test.ts verifying gap detection, clamping, alert firing, database recording, and non-re-alerting on subsequent cycles.

Closes #54

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[DevTobis]

Rebased onto main and slimmed. Main already clamps to the RPC's oldest ledger via getHealth(), returns skippedFrom/skippedTo and fires event_gap_skipped, so the oldestLedger passthrough, the indexer_gap alert and the gap-aware getLastIngestedLedger were dropped (they conflicted with the indexer cursor and duplicated the alert). Migration renumbered 0009 -> 0013. Kept: the chain_event_gap table, plus pollOnce now records the skipped range there (idempotent on the range index) with tests.