feat: add event indexer service for PrivacyLayer contracts

[HuiNeng6]

Summary

Implements #63 - Contract Events Indexer

This PR adds a comprehensive event indexer service for the PrivacyLayer smart contract, enabling efficient querying and real-time monitoring of all contract events.

Features

Event Indexing

• Real-time Event Indexing: Automatically polls Soroban RPC for new events
• All Event Types Supported: Deposit, Withdraw, PoolPaused, PoolUnpaused, VkUpdated
• Merkle Tree State Tracking: Maintains current Merkle root and leaf count

Storage

• PostgreSQL Database: Persistent storage with Prisma ORM
• Indexed Fields: Optimized queries on commitment, nullifierHash, recipient, timestamp, ledger
• Automatic Migrations: Prisma schema for easy database setup

GraphQL API

• Queries: Fetch deposits, withdrawals, admin events with filtering and pagination
• Subscriptions: Real-time updates via WebSocket
• Utility Queries: Check if commitment/nullifier is used, get indexer stats

Deployment

• Docker Support: Dockerfile and docker-compose.yml for easy deployment
• Environment Configuration: Flexible configuration via environment variables

Technical Stack

• Language: TypeScript
• API Framework: Apollo Server (GraphQL)
• Database: PostgreSQL with Prisma ORM
• Stellar Integration: soroban-client SDK
• Real-time: graphql-ws for WebSocket subscriptions

Example Usage

Query Recent Deposits:

query {
  deposits(pagination: { skip: 0, take: 10 }) {
    commitment
    leafIndex
    root
    timestamp
  }
}

Subscribe to New Deposits:

subscription {
  onDeposit {
    commitment
    leafIndex
    root
  }
}

Check if Commitment is Used:

query {
  isCommitmentUsed(commitment: "0x...")
}

Setup

1. Copy .env.example to .env and configure
2. Run migrations: npx prisma migrate dev
3. Start the indexer: npm run dev

Files Added

• indexer/src/index.ts - Main entry point
• indexer/src/indexer.ts - Event indexing logic
• indexer/src/parsers.ts - Event parsing utilities
• indexer/src/resolvers.ts - GraphQL resolvers
• indexer/src/schema.graphql - GraphQL schema
• indexer/prisma/schema.prisma - Database schema
• indexer/package.json, tsconfig.json, Dockerfile, docker-compose.yml, README.md

---

This implementation provides a solid foundation for off-chain event indexing that can be used by the SDK, frontend, and external integrations.

[HuiNeng6]

@ANAVHEOBA

📢 Follow-up — Ready for Review (24+ Hours)

This Event Indexer Service PR is ready for review.

Implementation Complete:
✅ MERGEABLE - Clean, ready to merge
✅ Event indexing - Contract event monitoring
✅ Database storage - Efficient event storage
✅ API endpoints - Query interface

Looking forward to your review! 🙏