feat: wire backend to deployed BNB Testnet contracts

[Copilot]

The backend had no live connection to the deployed GhostPool/GhostPaymaster contracts — contract addresses were placeholders, deposits required manual API calls, and nullifier/commitment checks were DB-only.

Contract ABI + Address Layer

• src/contracts/abis.ts — typed minimal ABIs for GhostPool and GhostPaymaster (view functions + events the backend actually calls)
• src/contracts/addresses.ts — chainId-keyed address registry hardcoded from deployments/97/addresses.json; env vars always override

Extended Contract Reads (paymasterClient.ts)

Added GhostPaymaster reads:

• isNullifierSpentOnChain(hash) — authoritative on-chain nullifier check
• getBnbToUsdcRate() — current fee conversion rate
• isZkVerificationEnabled() — paymaster verification flag

Refactored to use shared ABIs; exported publicClient for reuse.

On-Chain Deposit Indexer (src/indexer/depositIndexer.ts)

Polls GhostPool.Deposit events every 15s (configurable via INDEXER_POLL_INTERVAL_MS). Auto-inserts new deposits into the off-chain Merkle tree and DB — removes the requirement for the frontend to manually call POST /pool/deposit.

// First run looks back ~24h of blocks, then tracks from last processed block
const fromBlock = latestBlock > INITIAL_LOOKBACK_BLOCKS
  ? latestBlock - INITIAL_LOOKBACK_BLOCKS : 0n;

Route Hardening

• POST /pool/deposit — now calls isCommitmentInserted on-chain before accepting; rejects if commitment not yet in GhostPool
• GET /pool/status — response now includes onChain.merkleRoot and onChain.nextLeafIndex read live from the contract
• POST /relay — dual nullifier check: local DB (fast path) then isNullifierSpentOnChain (authoritative)

Config

• .env.example updated with BNB Testnet (chainId 97) deployed addresses pre-filled
• INDEXER_POLL_INTERVAL_MS env var added

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[OmLanke]

@Arnab-Afk Looks good to me. Please check