feat: Implement Reorg-Aware Event Indexer with zk-SNARK Quality Attestation Pipeline (#1319)

[Neelr1912]

Closes #1319

📌 Context & Problem Statement

Currently, CropChain relies on basic real-time event listeners which are vulnerable to state desynchronization from blockchain reorganizations (reorgs) and socket drops. Furthermore, confidential crop quality compliance data (pesticide PPM, moisture percentage, organic grading) needs to be attested on-chain without revealing sensitive farm trade secrets.

This PR implements a resilient Reorg-Aware Blockchain Event Indexer Engine combined with a Zero-Knowledge Proof (zk-SNARK) quality verification pipeline.

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🏗️ Summary of Changes

1. Circom zk-SNARK Quality Attestation Circuit (smart-contracts/circuits/)

• Created QualityAttestation.circom circuit to confidentially enforce:
  • Pesticide_PPM < Threshold
  • Moisture_Percentage within acceptable bounds [MIN, MAX]
  • Certified Grade == CertifiedGrade
• Created compile-circuit.js script to automate .wasm, .zkey, and Groth16 Solidity verifier artifact exports.

2. On-Chain zk-SNARK Verification & Replay Protection (smart-contracts/contracts/)

• Updated Verifier.sol with standard Groth16Verifier contract implementation.
• Updated CropChain.sol with verifyQualityAttestation() to validate ZK proofs before state transitions.
• Integrated spentProofHashes nullifier map to prevent proof replay attacks.

3. Reorg-Aware Event Indexer Engine (backend/)

• Created BlockHeader and IndexedEvent Mongoose models to track block parent hash continuity and event status (UNFINALIZED vs FINALIZED).
• Implemented configurable block confirmation depth ($N = 5$ blocks).
• Built automatic rollback to common ancestor block upon detecting chain forks up to 5 blocks deep.
• Implemented catch-up self-healing chunked block pagination (chunkSize = 100) via eth_getLogs for RPC recovery.

4. Dead Letter Queue (DLQ) & Admin Reconciliation API (backend/)

• Created DeadLetterEvent model and indexerDlqService.js to push unprocessable events to DLQ with exponential backoff retries.
• Exposed protected admin REST API POST /api/v1/indexer/reconcile for manual forced state synchronization.

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🧪 Edge Cases Handled

• 5-Block Chain Reorgs: Orphan records rolled back to common ancestor block and canonical chain re-indexed cleanly without leaving orphan DB state.
• Proof Replay Attacks: Spent ZK proof hashes tracked on-chain to revert duplicate proof submissions.
• RPC Failures & WS Disconnects: Chunked pagination catch-up recovers logs automatically after disconnection.

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✅ Testing & Verification

• Smart Contract Tests: 67/67 passing (npm run test:ci in smart-contracts/)
• Backend Indexer & DLQ Tests: 6/6 passing (npx jest tests/reorgIndexer.test.js)
• Circuit Build Pipeline: node scripts/compile-circuit.js executed cleanly.

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