[CT-01] Implement POST /submit in Rust verifier — anchor document hash to Stellar

[mftee]

Overview

The POST /submit endpoint in contract/src/lib.rs always returns 400 Bad Request with the body "Submit not yet implemented". This is the core function of the Rust microservice — anchoring a document hash to Stellar using a manageData operation. Without it, the Rust service has no production functionality.

Background

File: contract/src/stellar.rs — StellarClient struct, which already has verify_hash() and submit_transaction() as stubs.

Files to modify:

• contract/src/lib.rs — submit_handler() function (currently returns 400)
• contract/src/stellar.rs — StellarClient.anchor_hash() method (to be implemented)

Implementation approach:

1. Parse the request body: { hash: string, publicKey: string }
2. Build a Stellar transaction with a ManageData operation:
   • Key: "doc_" + &hash[..58] (max 64 bytes, matching NestJS buildDataKey())
   • Value: the full hash as bytes
3. Sign with the service's Stellar secret key (STELLAR_SECRET_KEY env var)
4. Submit to Stellar Horizon (stellar_sdk::Server.submit_transaction())
5. On success: cache the result in Redis (stellar:verify:{hash}) and return { txHash, ledger, anchoredAt }
6. On Stellar error: return 502 with the Horizon error detail

Stellar SDK: Use the stellar-sdk Rust crate or reqwest-based direct Horizon API calls.

Acceptance Criteria

• POST /submit with a valid hash and public key returns { txHash, ledger, anchoredAt } on success
• ManageData key format matches exactly what NestJS buildDataKey() produces (doc_ + first 58 chars of hash)
• Successful anchor result is cached in Redis to avoid duplicate submissions
• If the hash is already on Stellar (duplicate submission), return the existing record with 200 (idempotent)
• Returns 400 for missing or malformed hash
• Returns 502 with Horizon error details on Stellar network failure