Add Soroban event indexer for SmartDrop events

[ProtonsAndElectrons]

Closes #14

Summary

• add a Soroban RPC event poller for a configured SMARTDROP_CONTRACT_ID
• decode airdrop_created, recipient_added, token_claimed, and airdrop_expired events, storing raw XDR plus decoded data
• persist the last indexed ledger and aggregated airdrop/recipient/claim state through the existing Redis cache layer
• expose indexed status endpoints for airdrops, recipients, and indexer health
• document indexer environment variables and endpoints

Notes

This follows the backend's existing Redis persistence model instead of adding a new SQL dependency. The store is isolated behind src/indexer/eventStore.js, so it can be swapped for PostgreSQL later if the project adds a database layer.

Validation

• npm test -- --runInBand (59 tests passing)
• git diff --check

[prodbycorne]

fix merge conflicts