feat: implement persistent event indexer with sqlite storage (#77)

[Abdullahi-Code9]

Closes #583

📝 Description

This PR introduces a persistent event indexer (scripts/indexer.ts) and a CLI query tool (scripts/query-events.ts) backed by SQLite. It captures Soroban contract events, persists them locally for off-chain reconciliation and debugging, tracks progress across restarts, and provides instant CLI querying capabilities.

🚀 Changes Made

1. Persistent Event Indexer (scripts/indexer.ts)

• SQLite Persistence: Stores contract events in data/events.db using Node's native node:sqlite (DatabaseSync).
• Resumable Indexing: Stores the last indexed ledger sequence in indexer_meta. On service restart, indexing resumes seamlessly from the recorded checkpoint.
• Idempotent Storage & Batch Processing: Leverages explicit SQL transactions (BEGIN / COMMIT / ROLLBACK) for batch inserts while handling duplicate events idempotently using composite keys.

2. Event Query CLI Utility (scripts/query-events.ts)

• Implemented CLI query filters to query persisted events:
  • By account/address (--address)
  • By event type (--type)
  • By ledger range (--from-ledger, --to-ledger)

3. Type Safety & API Fixes

• Handled node:sqlite SQLOutputValue[] return types safely with explicit row interfaces.
• Maintained compatibility fixes for @stellar/stellar-sdk across helper scripts.

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🧪 Verification & Testing

Type Safety Verification

Verified strict TypeScript types across all scripts:

npx tsc --noEmit

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